VPP-62 Add a doxy filter to enable vpe.api doc

This makes Doxygen think the API definitions are structs which is close
enough to be able to document the API methods.

It also has logic to create an indexed API page but that's disabled for
now because it duplicates the "brief" text twice in the struct doc.

Fixes a minor line numbering issue in filter_c.py.

Change-Id: If380160b73e7c10d999b35a76f55d0e27cbc91cc
Signed-off-by: Chris Luke <chrisy@flirble.org>