mkcramfs; loff_t is guarded by _GNU_SOURCE

Although already properly done in cramfsck, it was missing in mkcramfs.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>

diff -durN cramfs-1.1.orig/mkcramfs.c cramfs-1.1/mkcramfs.c
--- cramfs-1.1.orig/mkcramfs.c	2015-08-05 22:31:16.830543096 +0200
+++ cramfs-1.1/mkcramfs.c	2015-08-05 22:33:33.136289744 +0200
@@ -22,6 +22,7 @@
  * If you change the disk format of cramfs, please update fs/cramfs/README.
  */
 
+#define _GNU_SOURCE
 #include <sys/types.h>
 #include <stdio.h>
 #include <sys/stat.h>