fs-util: include fcntl.h in fs-util.h

fs-util.h provides access_nofollow() but it did not include neither
fcntl.h nor unistd.h, which define F_OK and friends. Hence we cannot use
the function without including one of the headers. Let's include fcntl.h
in fs-util.h, then we can use the function by simply including fs-util.h.

src/basic/fs-util.h

@@ -1,6 +1,8 @@
 /* SPDX-License-Identifier: LGPL-2.1-or-later */
 #pragma once
 
+#include <fcntl.h>              /* IWYU pragma: export */
+
 #include "forward.h"
 #include "lock-util.h"