res_rtp_asterisk: Issue ERROR if res_srtp is not found.

While trying to get WebRTC working with chan_pjsip, I was running
into the following error:

    Attempted to set an invalid DTLS-SRTP configuration on RTP
    instance...

Josh helpfully pointed out that res_srtp.so might not be loaded, and
sure enough, it wasn't. This patch adds a ERROR indiciating as much
to hopefully help others having a similar problem.

Change-Id: I13aa477b47b299876728a21b130998a0ea6cd19f

res/res_rtp_asterisk.c

@@ -1256,6 +1256,7 @@ static int ast_rtp_dtls_set_configuration(struct ast_rtp_instance *instance, con
 	}
 
 	if (!ast_rtp_engine_srtp_is_registered()) {
+		ast_log(LOG_ERROR, "SRTP support module is not loaded or available. Try loading res_srtp.so.\n");
 		return -1;
 	}