scripts: get_abi: ignore code blocks for cross-references

The script should not generate cross-references inside
literal blocks.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/a590f994f8a5742db333bde69e88241a080e4fe0.1616668017.git.mchehab+huawei@kernel.org


Signed-off-by: Jonathan Corbet <corbet@lwn.net>