MB-7629 Fix warning on implicit cast

src/views/reductions.c: In function 'decode_view_btree_reductions':
src/views/reductions.c:48:8: warning: assignment discards 'const' qualifier from pointer target type [enabled by default]

Change-Id: Ia52e7dd666e7ccfa6b2414c1ad68182ee4568360
Reviewed-on: http://review.couchbase.org/25599
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Reviewed-by: Fulu Li <fulu@couchbase.com>
Tested-by: Filipe David Borba Manana <fdmanana@gmail.com>