abi_gen.sh: ignore gdb customisations when comparing signatures

abi_gen.sh: ignore gdb customisations when comparing signatures

If a .gdbinit file says "set print pretty on", the signatures are printed over
several lines, and the abi_check fails. So let's ignore .gdbinit files.

Signed-off-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Reviewed-by: David Disseldorp <ddiss@samba.org>
Reviewed-by: Andreas Schneider <asn@samba.org>
Autobuild-User(master): Andreas Schneider <asn@cryptomilk.org>
Autobuild-Date(master): Sat Feb  2 20:19:05 CET 2019 on sn-devel-144