Fix GCC compilation

include/grpc++/impl/call.h

@@ -98,9 +98,7 @@ class CallOpSendMessage {
   CallOpSendMessage() : send_buf_(nullptr), own_buf_(false) {}
 
   template <class M>
-  bool SendMessage(const M& message) GRPC_MUST_USE_RESULT {
-    return SerializationTraits<M>::Serialize(message, &send_buf_, &own_buf_);
-  }
+  bool SendMessage(const M& message) GRPC_MUST_USE_RESULT;
 
  protected:
   void AddOp(grpc_op* ops, size_t* nops) {
@@ -118,6 +116,11 @@ class CallOpSendMessage {
   bool own_buf_;
 };
 
+template <class M>
+bool CallOpSendMessage::SendMessage(const M& message) {
+  return SerializationTraits<M>::Serialize(message, &send_buf_, &own_buf_);
+}
+
 template <class R>
 class CallOpRecvMessage {
  public: