Added ifdef to GUID.cpp as the cocoapods are picking up this file and mac uses a function in GUID.mm.
Added ifdef to GUID.cpp as the cocoapods are picking up this file and
mac uses a function in GUID.mm.

getbraincloud/braincloud-cpp,getbraincloud/braincloud-cpp,getbraincloud/braincloud-cpp,getbraincloud/braincloud-cpp,getbraincloud/braincloud-cpp,getbraincloud/braincloud-cpp,getbraincloud/braincloud-cpp,getbraincloud/braincloud-cpp,getbraincloud/braincloud-cpp