The previous update introducing LFS support unconditionally changed the
sprintf() pattern used to print the file modification time to use PRIx64.

Explicitely convert the st_mtime member of the stat struct to uint64_t in
order to avoid type mismatch errors when building for non-64bit targets.

Signed-off-by: Jo-Philipp Wich <jow@openwrt.org>

SVN-Revision: 44138