Make the compiler even happier

src/video/SDL_surface.c

diff --git a/src/video/SDL_surface.c b/src/video/SDL_surface.c
index 47b6d2c..37740b2 100644
--- a/src/video/SDL_surface.c
+++ b/src/video/SDL_surface.c
@@ -42,7 +42,7 @@
     SDL_Surface *surface;
 
     /* The flags are no longer used, make the compiler happy */
-    flags;
+    (void)flags;
 
     /* Allocate the surface */
     surface = (SDL_Surface *) SDL_calloc(1, sizeof(*surface));