Add missing 'hostname-for-senders' attribute on the <gfe:gateway-receiver> element in the SDG XML namespace (XSD).

Currently, the Spring Data GemFire XML namespace (XSD) <gfe:gateway-receiver/> element is missing the 'hostname-for-senders' attribute, which allows developers to set the hostname the GatewaySenders will use to connect to the remote host (cluster) on the receiving side of the WAN Gateway.