Add #include "diff.h" in order to get the prototype for xmalloc().

The corresponding warning has been ignored for ages:
  prepend_args.c:75: warning: initialization makes pointer from integer
  without a cast
This is fatal on ia64.  You cannot convert an implicit int return from
an undeclared function to a pointer as you've lost the upper 32 bits.
On ia64, the warning is "pointer from different sized integer".

(This file is not vendor branched)

contrib/diff/prepend_args.c

@@ -23,6 +23,7 @@
 # include <config.h>
 #endif
 #include "system.h"
+#include "diff.h"
 #include "prepend_args.h"