chan_pjsip: Suppress frame warnings.

When rtp_keepalive is on for a PJSIP endpoint dialing to another
Asterisk instance also using PJSIP, Asterisk will continue to print
warning messages about not being able to send frames of a certain
type. This suppresses that warning message.

Change-Id: I0332a05519d7bda9cacfa26d433909ff1909be67

channels/chan_pjsip.c

@@ -941,6 +941,8 @@ static int chan_pjsip_write_stream(struct ast_channel *ast, int stream_num, stru
 			res = media->write_callback(session, media, frame);
 		}
 		break;
+	case AST_FRAME_CNG:
+		break;
 	default:
 		ast_log(LOG_WARNING, "Can't send %u type frames with PJSIP\n", frame->frametype);
 		break;