Makefile: skip reftable library for Coccinelle

The reftable library does not use any of the common helpers that the Git project has. Consequently, most of the rules that we have in Coccinelle do not apply to the library at all and may even generate false positives when a pattern can be converted to use a Git helper function. Exclude reftable library sources from being checked by Coccinelle to avoid such false positives. Signed-off-by: Patrick Steinhardt <ps@pks.im> Signed-off-by: Junio C Hamano <gitster@pobox.com>