BUILD FIX (r149718): Move exported symbol into ENABLE(FULLSCREEN_API) section

Fixes the following build failure when ENABLE(FULLSCREEN_API) is
off:

    Undefined symbols for architecture i386:
      "__ZNK7WebCore7Element25containsFullScreenElementEv", referenced from:
         -exported_symbol[s_list] command line option

* WebCore.exp.in: Move the symbol.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@149931 268f45cc-cd09-0410-ab3c-d52691b4dbfc