src/video_output/display.c: add return statement in non-void function.

The function VoutDisplayEventKeyDispatch() missed a return statement.

src/video_output/display.c

@@ -557,6 +557,7 @@ static void *VoutDisplayEventKeyDispatch(void *data)
 
         vlc_restorecancel(cancel);
     }
+    return NULL;
 }
 
 static void VoutDisplayEventKey(vout_display_t *vd, int key)