Allowing us to import google::protobuf::int64.

If for any reason, our definition of gpr_int64 is different from the definition of google::protobuf::int64, the virtual override would fail, as this needs to be of the precisely exactly the same type.
That change allows us to use protobuf's int64 definition.
diff --git a/include/grpc++/config.h b/include/grpc++/config.h
index fd76727..0267b85 100644
--- a/include/grpc++/config.h
+++ b/include/grpc++/config.h
@@ -42,9 +42,9 @@
 #define GRPC_OVERRIDE override
 #endif
 
-#ifndef GRPC_CUSTOM_STRING
-#include <string>
-#define GRPC_CUSTOM_STRING std::string
+#ifndef GRPC_CUSTOM_PROTOBUF_INT64
+#include <google/protobuf/stubs/common.h>
+#define GRPC_CUSTOM_PROTOBUF_INT64 ::google::protobuf::int64
 #endif
 
 #ifndef GRPC_CUSTOM_MESSAGE
@@ -52,6 +52,11 @@
 #define GRPC_CUSTOM_MESSAGE ::google::protobuf::Message
 #endif
 
+#ifndef GRPC_CUSTOM_STRING
+#include <string>
+#define GRPC_CUSTOM_STRING std::string
+#endif
+
 #ifndef GRPC_CUSTOM_ZEROCOPYOUTPUTSTREAM
 #include <google/protobuf/io/zero_copy_stream.h>
 #define GRPC_CUSTOM_ZEROCOPYOUTPUTSTREAM ::google::protobuf::io::ZeroCopyOutputStream
@@ -66,6 +71,7 @@
 namespace protobuf {
 
 typedef GRPC_CUSTOM_MESSAGE Message;
+typedef GRPC_CUSTOM_PROTOBUF_INT64 int64;
 
 namespace io {
 typedef GRPC_CUSTOM_ZEROCOPYOUTPUTSTREAM ZeroCopyOutputStream;