gtk-doc: don't regenerate gtk-doc in do_install

gtk-doc: don't regenerate gtk-doc in do_install

In out-of-tree builds gtk-doc's setup-build target copies all the content from
$srcdir to $builddir. However, if some of this content is regenerated at
configure time this can happen:

1) configure writes new build/version.xml
2) make compile copies content, including the tarball's src/version.xml
   to build/version.xml, and generates gtk-doc.
3) make install notices build/version.xml is older than configure.status,
   so regenerates gtk-doc.

gtk-doc generation is a slow process at the best of times, so doing it twice
isn't good.

(From OE-Core rev: 9ea9d61f26a942dbbc90070b40ba0209021a4b46)

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