[SDL] Fix missing header build error on some Linux systems.

[bruvzg]

I'm getting build errors on Asahi Linux (arm64) using GCC 15.1.1, but having stdlib included for free seems to be a generic C99+ requirement, so it's likely not specific to this config.

thirdparty/sdl/joystick/linux/SDL_sysjoystick.c:781:9: error: implicit declaration of function 'free' [-Wimplicit-function-declaration]
  781 |         free(entries[i]); // This should NOT be SDL_free()
      |         ^~~~
thirdparty/sdl/joystick/linux/SDL_sysjoystick.c:126:1: note: include '<stdlib.h>' or provide a declaration of 'free'
  125 | #include "../../core/linux/SDL_udev.h"

[Repiteo]

Thanks!