SDL_evdev.c: undefine _THIS before redefining it.

src/core/linux/SDL_evdev.c:104:1: warning: "_THIS" redefined
In file included from src/core/linux/../../events/SDL_events_c.h:26,
                 from src/core/linux/SDL_evdev.c:45:
src/core/linux/../../events/../video/SDL_sysvideo.h:146:1: warning: this is the location of the previous definition

src/core/linux/SDL_evdev.c

@@ -101,6 +101,7 @@ typedef struct SDL_EVDEV_PrivateData
     SDL_EVDEV_keyboard_state *kbd;
 } SDL_EVDEV_PrivateData;
 
+#undef _THIS
 #define _THIS SDL_EVDEV_PrivateData *_this
 static _THIS = NULL;