helper: wl-socket: fix build for musl

bind(2) expects its second parameters is a "const struct sockaddr*",
let's cast the "struct sockaddr_un*" to "struct sockaddr*" before
passing into bind(2).

SUN_LEN is a BSD extension that also implemented by GNU, let's define
_DEFAULT_SOURCE before including "sys/un.h".

src/helpers/wayland_wrapper/wl-socket.c

@@ -8,6 +8,7 @@
     SPDX-License-Identifier: GPL-2.0-or-later
 */
 
+#define _DEFAULT_SOURCE
 #include <assert.h>
 #include <errno.h>
 #include <fcntl.h>
@@ -155,7 +156,7 @@ struct wl_socket *wl_socket_create()
         s->fd = socket(PF_LOCAL, SOCK_STREAM, 0);
 
         int size = SUN_LEN(&s->addr);
-        int ret = bind(s->fd, &s->addr, size);
+        int ret = bind(s->fd, (struct sockaddr*)&s->addr, size);
         if (ret < 0) {
             goto fail;
         }