selftests: net: suppress ReST file generation when building selftests

Some selftests need libynl.a. When building it try to skip
generating the ReST documentation, libynl.a does not depend
on them.

Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20250203214850.1282291-1-kuba@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>