base-call: Increase incoming call timeout to ten seconds

The standard says a RING or CRING should be emitted whenever the
network sends an incoming call notification, which is nation-specific:

  "Interpretation of indications from the network to determine what
  constitutes a "ring" is defined by national regulations.  This
  result code should be repeated each time the network repeats the
  incoming call indication."  -- ITU-T Rec. V.250, p. 42

On the giffgaff (O2) network in the UK, a SIMCom SIM7100E modem emits
RING indicators every six seconds.  The current timeout is only five
seconds resulting in a stream of timed-out ModemManager call objects
whenever there is an incoming call.  To fix this, we increase the
timeout to ten seconds.

src/mm-base-call.c

@@ -147,7 +147,7 @@ cleanup_unsolicited_events (MMBaseCall  *self,
  * then we assume the call attempt is finished and we transition to TERMINATED.
  */
 
-#define INCOMING_TIMEOUT_SECS 5
+#define INCOMING_TIMEOUT_SECS 10
 
 static gboolean
 incoming_timeout_cb (MMBaseCall *self)