staging: add ${datadir}/gtk-doc/html to the sysroot blacklist

staging: add ${datadir}/gtk-doc/html to the sysroot blacklist

When api-documentation is enabled the GNOME stack builds API documentation. As
${datadir} is in SYSROOT_DIRS this documentation is in the sysroot but is never
used, wasting time and space.

Add ${datadir}/gtk-doc/html to the blacklist so that the generated documentation
isn't in the sysroot.  Note that we don't blacklist all of ${datadir}/gtk-doc
because gtk-doc itself installs files there which are needed to use gtk-doc.

(From OE-Core rev: 11c87952c9a71036119d509ce09f17e352e4c6d8)

Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>