Add missing prototype for get_hold_on_other_key_press to resolve #18855

[nicknimchuk]

Description

Adds the missing function prototype for get_hold_on_other_key_press in /quantum/action_tapping.h. The missing prototype doesn't cause errors most of the time, but does (at least) if both auto-shift and HOLD_ON_OTHER_KEY_PRESS_PER_KEY are enabled, due to use of get_hold_on_other_key_press in quantum/process_keycode/process_auto_shift.c.

Types of Changes

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

Issues Fixed or Closed by This PR

• Closes [Bug] Cannot enable auto-shift and HOLD_ON_OTHER_KEY_PRESS_PER_KEY at the same time #18855

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).