[GTK] Move image decoders, some other GStreamer, Cairo and Soup sources to libPlatform
https://bugs.webkit.org/show_bug.cgi?id=115050

Reviewed by Martin Robinson.

Source/Platform: 

* GNUmakefile.am: Add the image-decoders directory and its subdirectories to the list of header inclusion directories.
List the Cairo, GStreamer and Libsoup cppflags under the cppflags libPlatform uses for compiling its sources.

Source/WebCore: 

No new tests - no new functionality.

* GNUmakefile.list.am: Move the image decoders source files and a couple of GStreamer, Cairo and Libsoup source
files under the libPlatform's list of files to build. All of these are already free of platform layer violations.


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@149018 268f45cc-cd09-0410-ab3c-d52691b4dbfc