xfstests: fix compile error of src/fssum.c on bigendian host

Definition of htonll() is needed on bigendian host too, otherwise
src/fssum.c won't compile on ppc64/s390x hosts.

Acked-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Carlos Maiolino <cmaiolino@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>
diff --git a/fssum.c b/fssum.c
index f57594c..948bc35 100644
--- a/fssum.c
+++ b/fssum.c
@@ -43,6 +43,8 @@
 #ifdef __LINUX__
 #if __BYTE_ORDER == __LITTLE_ENDIAN
 #define htonll(x)     __bswap_64 (x)
+#else
+#define htonll(x)     (x)
 #endif
 #endif