[CMake] Consolidate list of files to build for WebCore plugin support
https://bugs.webkit.org/show_bug.cgi?id=104429

Reviewed by Rob Buis.

Move the common files to support plugins and the logic for disabling
plugins) to CMakeLists.txt from PlatformXXX.cmake.

No new tests as there is no new functionality.

* CMakeLists.txt:
* PlatformBlackBerry.cmake:
* PlatformEfl.cmake:
* PlatformWinCE.cmake:


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@149899 268f45cc-cd09-0410-ab3c-d52691b4dbfc