gtk-doc.bbclass: unbreak dependency-chain for native recipes

gtk-doc.bbclass: unbreak dependency-chain for native recipes

see [1] & [2] for discussion of this patch

[1] http://lists.linuxtogo.org/pipermail/openembedded-core/2012-July/026606.html
[2] http://lists.linuxtogo.org/pipermail/openembedded-core/2012-July/026611.html

(From OE-Core rev: f9ae930552bc5f1d59f207d4cd0e2b1b4f811dbc)

Signed-off-by: Andreas Müller <schnitzeltony@googlemail.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>