Fix SDL2 GFX Suspend Screensaver Warning

[RobLoach]

When building RetroArch and X11 is available, it will skip using sdl2_gfx_suspend_screensaver(), resulting in a warning. This change cleans up the declaration code, and makes the macro condition within the function itself, so that we no longer get the warning when building.

gfx/drivers/sdl2_gfx.c:542:13: warning: ‘sdl2_gfx_suspend_screensaver’ defined but not used [-Wunused-function]
  542 | static bool sdl2_gfx_suspend_screensaver(void *data, bool enable) { return false; }
      |             ^~~~~~~~~~~~~~~~~~~~~~~~~~~~

[LibretroAdmin]

This should achieve the same purpose I hope -
466e82f

I'd rather not deal with useless wrapper functions.

[RobLoach]

Thanks!