docs: kernel_abi.py: use --enable-lineno for get_abi.pl

docs: kernel_abi.py: use --enable-lineno for get_abi.pl

Just like kernel-doc extension, we need to be able to identify
what part of an imported document has issues, as reporting them
as:

	get_abi.pl rest --dir $srctree/Documentation/ABI/obsolete --rst-source:1689: ERROR: Unexpected indentation.

Makes a lot harder for someone to fix.

It should be noticed that it the line which will be reported is
the line where the "What:" definition is, and not the line
with actually has an error.

Acked-by: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/d6155ab16fb7631f2fa8e7a770eae72f24bf7cc5.1604042072.git.mchehab+huawei@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>