xfstests: ensure uint64_t is defined for <linux/fs.h>

When compiling "fiemap-tester.c" in my environment, I am
getting complaints at the first reference to "uint64_t"
in <linux/fs.h>.  This simple patch resolves that.

Signed-off-by: Alex Elder <aelder@sgi.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>

src/fiemap-tester.c

@@ -18,6 +18,7 @@
 
 #include <stdio.h>
 #include <stdlib.h>
+#include <stdint.h>
 #include <unistd.h>
 #include <fcntl.h>
 #include <string.h>