Have Cocoapods rename includes in the core wrapper

Fixes https://github.com/grpc/grpc/issues/2194 .
diff --git a/src/objective-c/GRPCClient/GRPCCall.m b/src/objective-c/GRPCClient/GRPCCall.m
index a9625a1..77eebef 100644
--- a/src/objective-c/GRPCClient/GRPCCall.m
+++ b/src/objective-c/GRPCClient/GRPCCall.m
@@ -34,7 +34,7 @@
 #import "GRPCCall.h"
 
 #include <grpc/grpc.h>
-#include <grpc/support/grpc_time.h>
+#include <grpc/support/time.h>
 
 #import "GRPCMethodName.h"
 #import "private/GRPCChannel.h"