test/statx: be safe and check if __NR_statx is there

Probably don't need this, but it's right before release and I'd hate
for a test to cause compilation failure.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
diff --git a/test/statx.c b/test/statx.c
index d354d19..66f333f 100644
--- a/test/statx.c
+++ b/test/statx.c
@@ -15,11 +15,20 @@
 
 #include "liburing.h"
 
+#ifdef __NR_statx
 static int do_statx(int dfd, const char *path, int flags, unsigned mask,
 		    struct statx *statxbuf)
 {
 	return syscall(__NR_statx, dfd, path, flags, mask, statxbuf);
 }
+#else
+static int do_statx(int dfd, const char *path, int flags, unsigned mask,
+		    struct statx *statxbuf)
+{
+	errno = ENOSYS;
+	return -1;
+}
+#endif
 
 static int create_file(const char *file, size_t size)
 {