dtmf: support Flash (!) signal

Refs #51007
Signed-off-by: Tristan Matthews <tristan.matthews@savoirfairelinux.com>

Change-Id: Id75356b6894ef227032c6c447c7df3b6436ae1bc

daemon/src/audio/audiortp/dtmf_event.cpp

@@ -47,6 +47,10 @@ DTMFEvent::DTMFEvent(char digit) : payload(), newevent(true), length(1000)
     */
 
     switch (digit) {
+        case '!':
+            digit = 16;
+            break;
+
         case '*':
             digit = 10;
             break;