[GTK] libPlatform, libPlatformGtk must depend on the related GNUmakefiles
https://bugs.webkit.org/show_bug.cgi?id=115937

Reviewed by Martin Robinson.

Source/Platform: 

* GNUmakefile.am: Set up an artifical dependency so that libPlatform and libPlatformGtk
libraries take into account any changes in this GNUmakefile.am and the GNUmakefile.list.am
located under Source/WebCore. The latter is included due to all the build targets for the
two libraries being listed there.

Source/WebCore: 

No new tests - no new functionality.

* GNUmakefile.am: Fix typos in two libraries' names - Webcore -> WebCore.


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@150073 268f45cc-cd09-0410-ab3c-d52691b4dbfc