musl: introduce dummy gshadow header file for userdb

Even 'gshadow' meson option is disabled, src/shared/userdb.c and
src/shared/user-record-nss.c include gshadow.h unconditionally.
Let's introduce dummy header to make them compiled gracefully.

src/include/musl/gshadow.h

@@ -0,0 +1,22 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+#pragma once
+
+#include <errno.h>
+#include <stddef.h>
+
+struct sgrp {
+        char *sg_namp;
+        char *sg_passwd;
+        char **sg_adm;
+        char **sg_mem;
+};
+
+static inline int getsgnam_r(
+                const char *__name,
+                struct sgrp *__result_buf,
+                char *__buffer,
+                size_t __buflen,
+                struct sgrp **__result) {
+
+        return EOPNOTSUPP; /* this function returns positive errno in case of error. */
+}