Increase permissions on setCommon

By being private classes could only easily copy data as far as was
defined in AntResponse. By decreasing it allows the ANTPLUS repo to
continue subclassing into its datapages while maintaining the
original message data.

src/RX/ANT_AntResponse.h

@@ -136,8 +136,8 @@ class AntResponse {
 protected:
  // pointer to frameData
  uint8_t* _frameDataPtr;
-private:
  void setCommon(AntResponse &target);
+private:
  uint8_t _msgId;
  uint8_t _length;
  uint8_t _checksum;