ci: build and use libabigail 1.6

ci: build and use libabigail 1.6

libabigail 1.2 (at least) reports changes in 'const' property as an ABI
breakage [1].
This was fixed upstream in libabigail 1.4 [2], and a bug has been opened
in launchpad [3].

But for now, build and use the last version 1.6 so that the ABI checks
can be kept.

1: https://travis-ci.com/DPDK/dpdk/jobs/287872118#L2242
2: https://sourceware.org/git/gitweb.cgi?p=libabigail.git;a=commitdiff;h=215b7eb4fe8b
3: https://bugs.launchpad.net/ubuntu/+source/libabigail/+bug/1863607

Signed-off-by: David Marchand <david.marchand@redhat.com>
Acked-by: Thomas Monjalon <thomas@monjalon.net>
Acked-by: Aaron Conole <aconole@redhat.com>