Add explicit cast to fix buildbreak
Add explicit cast to fix buildbreak

--enable-warnings didn't like the return of const** when
the prototype said const char * const *... Add and explicit
cast..

Change-Id: I1ff18503c931a1c85a47bb70e239747bc1fb8726
Reviewed-on: http://review.couchbase.org/20434
Tested-by: Trond Norbye <60edd2ef23891a753f231b0c6f161dc634079a93@gmail.com>
Reviewed-by: Sergey Avseyev <87f6d5e4fd3644c3c20800cde7fd3ad1569370b3@gmail.com>

maxim-ky/libcouchbase,signmotion/libcouchbase,senthilkumaranb/libcouchbase,mnunberg/libcouchbase,uvenum/libcouchbase,senthilkumaranb/libcouchbase,couchbase/libcouchbase,avsej/libcouchbase,PureSwift/libcouchbase,trondn/libcouchbase,couchbase/libcouchbase,uvenum/libcouchbase,uvenum/libcouchbase,uvenum/libcouchbase,mnunberg/libcouchbase,couchbase/libcouchbase,kojiromike/libcouchbase,kojiromike/libcouchbase,trondn/libcouchbase,avsej/libcouchbase,senthilkumaranb/libcouchbase,couchbase/libcouchbase,trondn/libcouchbase,avsej/libcouchbase,mody/libcouchbase,mnunberg/libcouchbase,senthilkumaranb/libcouchbase,signmotion/libcouchbase,avsej/libcouchbase,maxim-ky/libcouchbase,trondn/libcouchbase,avsej/libcouchbase,maxim-ky/libcouchbase,mnunberg/libcouchbase,couchbase/libcouchbase,mnunberg/libcouchbase,maxim-ky/libcouchbase,signmotion/libcouchbase,PureSwift/libcouchbase,signmotion/libcouchbase,uvenum/libcouchbase,mody/libcouchbase,kojiromike/libcouchbase,trondn/libcouchbase,avsej/libcouchbase,couchbase/libcouchbase,mody/libcouchbase,PureSwift/libcouchbase,kojiromike/libcouchbase,PureSwift/libcouchbase,couchbase/libcouchbase,avsej/libcouchbase,mody/libcouchbase