callback.c: windows build fix
callback.c: windows build fix

Change-Id: I4a670d8d476c0c4503faee38235362670934b781
Reviewed-on: http://review.couchbase.org/38897
Tested-by: Mark Nunberg <e7d42768707bf23038325b0b68d4b577e5f6064a@haskalah.org>
Reviewed-by: Brett Lawson <c8c4369773d7d7226e031f0c4af62e3f38a08510@gmail.com>

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