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".

(cherry picked from commit 7800f155)