scripts: get_api.pl: Add sub-titles to ABI output

Instead of adding titles just for the files, add titles
for each part of the ABI output, in order to make easier
to search for a symbol there.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/64752a5de06ab8263c296e3ed01414b25861e1eb.1604312590.git.mchehab+huawei@kernel.org


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