MB-8746: Additional export for set view based spatial index

Export couch_set_view:modify_bitmasks/2 which will be used
by the set view based spatial index.

Change-Id: I5efcab378e80b3949861d725620a9f2bbea09ace
Reviewed-on: http://review.couchbase.org/28064
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: Volker Mische <volker.mische@gmail.com>