stasis/control.c: Set Hangup Cause to No Answer on Dial timeout

stasis/control.c: Set Hangup Cause to No Answer on Dial timeout

Other Dial operations (dial, app_dial) use Q.850 cause 19 when a dial timeout occurs,
but the Dial command via ARI did not set an explicit reason. This resulted in a
CANCEL with Normal Call Clearing and corresponding ChannelDestroyed.

This change sets the hangup cause to AST_CAUSE_NO_ANSWER to be consistent with the
other operations.

Fixes: #963
UserNote:  A Dial timeout on POST /channels/{channelId}/dial will now result in a
CANCEL and ChannelDestroyed with cause 19 / User alerting, no answer.  Previously
no explicit cause was set, resulting in a cause of 16 / Normal Call Clearing.