Unreviewed build fix. libWebCore.la needs to be relinked when
symbols.filter changes.

* GNUmakefile.am: add symbols.filter as a dependency for the
libWebCore.la library.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@142108 268f45cc-cd09-0410-ab3c-d52691b4dbfc