Userland: Properly define IPC::encode and IPC::decode specializations

In order to avoid the base encode/decode methods from being used (and
failing a static assertion), we must be sure to declare/define the
custom type implementations as template specializations.

After this, LibIPC is no longer sensitive to include order.

Userland/Libraries/LibWeb/Cookie/Cookie.h

@@ -46,7 +46,10 @@ SameSite same_site_from_string(StringView same_site_mode);
 
 namespace IPC {
 
+template<>
 bool encode(Encoder&, Web::Cookie::Cookie const&);
+
+template<>
 ErrorOr<void> decode(Decoder&, Web::Cookie::Cookie&);
 
 }