Fix sdl_zoom compile problems on OpenBSD

Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
diff --git a/sdl_zoom.c b/sdl_zoom.c
index 56d3604..a986c7c 100644
--- a/sdl_zoom.c
+++ b/sdl_zoom.c
@@ -14,6 +14,7 @@
 #include "sdl_zoom.h"
 #include "osdep.h"
 #include <stdint.h>
+#include <stdio.h>
 
 static int sdl_zoom_rgb16(SDL_Surface *src, SDL_Surface *dst, int smooth,
                           SDL_Rect *dst_rect);