Set mod_voicemail to depend on mail-transport-agent

Without this dependency we might not end up with sendmail installed
which would be unnecessarily surprising when using mod_voicemail.

debian/control-modules

@@ -227,6 +227,7 @@ Description: Voicemail detection
  This module detects voicemail beeps at any frequency in O(1) time.
 
 Module: applications/mod_voicemail
+Depends: mail-transport-agent
 Description: Voicemail
  This module provides a voicemail system.