docs: abi-testing.rst: enable --rst-sources when building docs

Now that ABI/testing documents were fixed, add --rst-sources to
the ABI/testing too.

Acked-by: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/9242473fd3df785565bb6084b1b814cc15074fb2.1604042072.git.mchehab+huawei@kernel.org


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>