Stripped C standard library headers (and a couple of macros)

from the tools code. This is all handled in H5private.h, which
should be included in all tools source files.

tools/src/h5diff/h5diff_common.c

@@ -11,8 +11,6 @@
  * help@hdfgroup.org.                                                        *
  * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * */
 
-#include <stdlib.h>
-#include <string.h>
 #include "H5private.h"
 #include "h5diff.h"
 #include "h5diff_common.h"