Include <stddef.h> where ptrdiff_t is used.

Signed-off-by: Andrew Borodin <aborodin@vmail.ru>

lib/util.c

@@ -35,6 +35,7 @@
 #include <config.h>
 
 #include <ctype.h>
+#include <stddef.h>             /* ptrdiff_t */
 #include <limits.h>
 #include <stdarg.h>
 #include <stdio.h>

src/diffviewer/ydiff.c

@@ -28,8 +28,10 @@
 
 
 #include <config.h>
+
 #include <ctype.h>
 #include <errno.h>
+#include <stddef.h>             /* ptrdiff_t */
 #include <stdlib.h>
 #include <sys/stat.h>
 #include <sys/types.h>