utimensat01: Fix build for musl

which requires _GNU_SOURCE to have loff_t.

Fixes: f8cbbad ("syscalls/utimensat: Migrate to new test framework")

Signed-off-by: Petr Vorel <[email protected]>

Author: pevik

testcases/kernel/syscalls/utimensat/utimensat01.c

@@ -7,6 +7,7 @@
  * Basic utimnsat() test.
  */
 
+#define _GNU_SOURCE
 #include <stdio.h>
 #include <time.h>
 #include <errno.h>