cbfstool: compare pointer difference with ptrdiff_t value

Fixes building cbfstool in 32bit environments.

Change-Id: I3c94afc9c961eb8b41d1e08f4a16e5cab2a6bb8b
Signed-off-by: Patrick Georgi <pgeorgi@google.com>
Reviewed-on: http://review.coreboot.org/10015
Tested-by: build bot (Jenkins)
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>
diff --git a/util/cbfstool/cbfs_image.c b/util/cbfstool/cbfs_image.c
index e61664c..2581bef 100644
--- a/util/cbfstool/cbfs_image.c
+++ b/util/cbfstool/cbfs_image.c
@@ -19,6 +19,7 @@
 
 #include <inttypes.h>
 #include <libgen.h>
+#include <stddef.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
@@ -449,7 +450,7 @@
 	      content_offset, (int)((char*)CBFS_SUBHEADER(entry) -
 				    image->buffer.data));
 	assert((char*)CBFS_SUBHEADER(entry) - image->buffer.data ==
-	       content_offset);
+	       (ptrdiff_t)content_offset);
 	memcpy(CBFS_SUBHEADER(entry), data, size);
 	if (verbose > 1) cbfs_print_entry_info(image, entry, stderr);