Expose garbageCollectNow()

This is a temporary workaround. Application developer should be able
to trigger the GC by creating a lot of purpose-less Objects.
Fixed linker error with GStreamer. https://bugs.webkit.org/show_bug.cgi?id=124861

Patch by Alex Christensen <achristensen@webkit.org> on 2013-12-17
Reviewed by Martin Robinson.

* wtf/gobject/GOwnPtr.cpp:
(WTF::GError):
* wtf/gobject/GOwnPtr.h:
Added WTF_EXPORT_PRIVATE to freeOwnedGPtr<GError> declaration and definition.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@160716 268f45cc-cd09-0410-ab3c-d52691b4dbfc