LookupServer: Use designated initializers for sockaddr_in

At least macOS has a non-standard sin_len field at the front of
the struct that Linux and Serenity do not. On BSDs, the
sin_len field must be initialized to the size of the structure.

Co-Authored-By: Timon Kruiper <timonkruiper@gmail.com>

Userland/Services/LookupServer/MulticastDNS.h

@@ -36,12 +36,15 @@ private:
     Name m_hostname;
 
     static constexpr sockaddr_in mdns_addr {
-        AF_INET,
+#ifdef AK_OS_BSD_GENERIC
+        .sin_len = sizeof(struct sockaddr_in),
+#endif
+        .sin_family = AF_INET,
         // htons(5353)
-        0xe914,
+        .sin_port = 0xe914,
         // 224.0.0.251
-        { 0xfb0000e0 },
-        { 0 }
+        .sin_addr = { 0xfb0000e0 },
+        .sin_zero = { 0 }
     };
 };