THRIFT-2180 Integer types issues in Cocoa lib on ARM64

CLient: Cocoa
Patch: Jeremy W. Sherman
This closes #372
diff --git a/lib/cocoa/src/transport/THTTPClient.h b/lib/cocoa/src/transport/THTTPClient.h
index 4d57840..78935fb 100644
--- a/lib/cocoa/src/transport/THTTPClient.h
+++ b/lib/cocoa/src/transport/THTTPClient.h
@@ -25,7 +25,7 @@
   NSMutableURLRequest * mRequest;
   NSMutableData * mRequestData;
   NSData * mResponseData;
-  int mResponseDataOffset;
+  size_t mResponseDataOffset;
   NSString * mUserAgent;
   int mTimeout;
 }