Fix the "redundant redeclaration" warning for __debugbreak when using MinGW

[rfomin]

The MinGW header implementation of __debugbreak triggers a warning when built with the "-Wredundant-decls" option. This has been tested with MSYS2 UCRT64 GCC.

Test program:

#include <stdlib.h>
#include <SDL3/SDL.h>
#include <SDL3/SDL_main.h>
int main(int argc, char **argv)
{
    return 0;
}

Existing Issue(s)

#14263