libgtk2: ensure faq/tutorial isn't regenerated

As we don't need them, the tarball already contains generated documentation
and it fails to build with certain versions of db2html:

http://article.gmane.org/gmane.comp.lib.uclibc.buildroot/65165



Reported-by: Justin Swartz <justin.swartz@nym.hush.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>