[macOS] Building on M1/ Apple Silicon/ Arm - SDL2 headers location needs to be altered

[shinra-electric]

Building on the M1 fails with SDL2 not being located.

I had come across this issue before on another project, and fixed it by removing the SDL2/ directory when adding an include.

For example, include "SDL2/SDL.h" should be changed to include "SDL.h"

I did this for the following files until it built:

isr.c modexlib.c rt_in.c rt_main.c rt_menu.c rt_uitl.c dukemusc.c audiolib/dsl.c

Note some of the above files had multiple SDL2 references.

Also there may be other references elsewhere, but just changing the above fixes the default build at least.