Fix helix/rev2 not working properly when using split_common.

[mtei]

Description

When helix/rev2 was using split_common, it didn't work properly and I couldn't type on the right hand side.

The cause is that the following code, added in 0.16.0 to quantum/keyboard.c, does not include quantum/split_common/split_util.h but instead includes keyboards/helix/rev2/split_util.h. Therefore, split_pre_init()/split_post_init() in quantum/split_common/split_util.c was not called.

  #ifdef SPLIT_KEYBOARD
  #    include "split_util.h"
  #endif

 % make clean; make VERBOSE_C_INCLUDE=keyboard.c helix/rev2/back:default
QMK Firmware 0.16.0
Deleting .build/ ... done.
QMK Firmware 0.16.0
Making helix/rev2/back with keymap default

avr-gcc (Homebrew AVR GCC 8.4.0_2) 8.4.0
Copyright (C) 2018 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

Compiling: keyboards/helix/rev2/keymaps/default/oled_display.c                                      [OK]

.......................
Compiling: quantum/keyboard.c
. quantum/quantum.h
.. platforms/avr/platform_deps.h
... /usr/local/Cellar/avr-gcc@8/8.4.0_2/avr/include/avr/pgmspace.h
.... /usr/local/Cellar/avr-gcc@8/8.4.0_2/lib/avr-gcc/8/gcc/avr/8.4.0/include/stddef.h
... /usr/local/Cellar/avr-gcc@8/8.4.0_2/avr/include/avr/interrupt.h
.. platforms/wait.h
.......................
. quantum/bootmagic/magic.h
. quantum/command.h
. quantum/crc.h
. keyboards/helix/rev2/split_util.h ◆◆◆◆◆ LOOK ◆◆◆◆◆
Multiple include guards may be useful for:
./quantum/rgblight/rgblight_post_config.h
./quantum/split_common/post_config.h
/usr/local/Cellar/avr-gcc@8/8.4.0_2/avr/include/avr/iom32u4.h
quantum/rgblight/rgblight_modes.h
.......................
Compiling: lib/lufa/LUFA/Drivers/USB/Core/USBTask.c                                                 [OK]
Compiling: tmk_core/protocol/lufa/usb_util.c                                                        [OK]
Linking: .build/helix_rev2_back_default.elf                                                         [OK]
Creating load file for flashing: .build/helix_rev2_back_default.hex                                 [OK]
Copying helix_rev2_back_default.hex to qmk_firmware folder                                          [OK]
Checking file size of helix_rev2_back_default.hex                                                   [OK]
 * The firmware size is fine - 26054/28672 (90%, 2618 bytes free)

Types of Changes

• Core
• Bugfix
• New feature
• Enhancement/optimization
• Keyboard (addition or update)
• Keymap/layout/userspace (addition or update)
• Documentation

Checklist

• My code follows the code style of this project: C, Python
• I have read the PR Checklist document and have made the appropriate changes.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• I have tested the changes and verified that they work and don't break anything (as well as I can manage).

[zvecr]

The cause is that the following code, added in 0.16.0

For completeness, it was the merge of #16432 that is the cause of the issue. It probably should have been rebased/retested after the develop merge.