quota-util: explicitly include linux/quota.h

The header linux/quota.h provides e.g. QIF_DQBLKSIZE or PRJQUOTA, which
is used where the quota-util.h is included.
Let's explicitly include the header with 'IWYU pragma: export' tag.

src/shared/quota-util.h

@@ -1,7 +1,8 @@
 /* SPDX-License-Identifier: LGPL-2.1-or-later */
 #pragma once
 
-#include <sys/quota.h> /* IWYU pragma: export */
+#include <linux/quota.h>        /* IWYU pragma: export */
+#include <sys/quota.h>          /* IWYU pragma: export */
 
 #include "forward.h"