Make src/system use pragma once in headers

[andy31415]

Problem

Include header guards are more typing, pollute the global define space, have a chance to mistmatch ending comments (if we ever rename and copy&paste).

#pragma once is supported most compilers already, let's use it.

Summary of Changes

uses #pragma once in headers under src/system

parth of #3138

[github-actions]

Size increase report for "esp32-example-build"

File	Section	File	VM
chip-wifi-echo.elf	.flash.rodata	8	8
chip-wifi-echo.elf	.dram0.bss	0	-8
chip-wifi-echo.elf	.dram0.data	-8	-8

Full report output

BLOAT REPORT

Files found only in the build output:
    report.csv

Comparing ./master_artifact/chip-wifi-echo.elf and ./pull_artifact/chip-wifi-echo.elf:

sections,vmsize,filesize
.flash.rodata,8,8
.debug_abbrev,0,2
.debug_info,0,1
[Unmapped],0,-3
.dram0.bss,-8,0
.dram0.data,-8,-8