[Gtk] [WK2] Fix build after r141619
https://bugs.webkit.org/show_bug.cgi?id=108687

Reviewed by Benjamin Poulain.

Take a reference instead of a pointer for decoding functions.

* Platform/gtk/SharedMemoryGtk.cpp:
(WebKit::SharedMemory::Handle::decode):
* Shared/gtk/ArgumentCodersGtk.cpp:
(CoreIPC::decodeImage):
(CoreIPC::decodeDataObject):
(CoreIPC::::decode):
(CoreIPC::decodeGKeyFile):
(CoreIPC::decode):
* Shared/gtk/ArgumentCodersGtk.h:
* Shared/gtk/LayerTreeContextGtk.cpp:
(WebKit::LayerTreeContext::decode):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@141640 268f45cc-cd09-0410-ab3c-d52691b4dbfc