Fix: Build error on headless build

This simple patch removes an "UNUSED_VARS" macro referencing
a variable which doesn't exist (r_unit_size).

It only affects the headless build

Differential Revision: https://developer.blender.org/D7464
Reviewed By: harley

source/blender/windowmanager/intern/wm_splash_screen.c

@@ -254,7 +254,6 @@ static ImBuf *wm_block_splash_image(int width, int *r_height)
 
   return ibuf;
 #else
-  UNUSED_VARS(r_unit_size);
   return NULL;
 #endif
 }