[master] qualify a namespace within InMemoryZoneFinder more strictly.

without this SunStudio will be confused about the namespace for the
declaration of internal Context below and refuse to compile.

confirmed it works, okayed on jabber.

src/lib/datasrc/memory/zone_finder.h

@@ -66,7 +66,7 @@ public:
     /// This specialized version exploits internal data structure to find
     /// RRsets at the zone origin and (if \c use_minttl is true) extract
     /// the SOA Minimum TTL much more efficiently.
-    virtual boost::shared_ptr<Context> findAtOrigin(
+    virtual boost::shared_ptr<ZoneFinder::Context> findAtOrigin(
         const isc::dns::RRType& type, bool use_minttl,
         FindOptions options);