Compile fix for gnutls-3.0.5

[Simon Josefsson]

Trying to build trunk, I also noticed that libgnutls-extra is still
referenced by the GTK-DOC manual...

gtk-doc: Building HTML
warning: failed to load external entity "../xml/extra.xml"
../gnutls-docs.sgml:25: element include: XInclude error : could not load ../xml/extra.xml, and no fallback was found
warning: failed to load external entity "../xml/openssl.xml"
../gnutls-docs.sgml:32: element include: XInclude error : could not load ../xml/openssl.xml, and no fallback was found
Computing chunks...

/Simon