vo_opengl_old: add missing include statement

<ctype.h> is needed at least for isalnum(). Most time this worked,
because some ffmpeg or Libav versions recursively include this header
from libavutil/common.h. Fix it so it always works.

video/out/vo_opengl_old.c

@@ -26,6 +26,7 @@
 #include <string.h>
 #include <math.h>
 #include <stdbool.h>
+#include <ctype.h>
 #include <assert.h>
 
 #include "config.h"