Set visibility("default") for GCC on exported symbols

This does nothing with the current configuration, but it's useful if you compile the library with -fvisibility=hidden

Patch by Modestas Vainius


git-svn-id: svn://anonsvn.kde.org/home/kde/trunk/kdesupport/taglib@1039693 283d02a7-25f6-0310-bc7c-ecb5cbfe19da