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/LibGfx/Point.cpp

@@ -51,12 +51,14 @@ String FloatPoint::to_string() const
 
 namespace IPC {
 
+template<>
 bool encode(Encoder& encoder, Gfx::IntPoint const& point)
 {
     encoder << point.x() << point.y();
     return true;
 }
 
+template<>
 ErrorOr<void> decode(Decoder& decoder, Gfx::IntPoint& point)
 {
     int x = 0;