simtech: handle non-standard '+CRING' URCs

The SIM7600E ends up emitting these URCs with too many <CR>s, and the
generic +CRING handler doesn't catch them, interfering with other
actions, e.g.:

  $ sudo mmcli --call 1 --accept
  error: couldn't accept the call: 'GDBus.Error:org.freedesktop.ModemManager1.Error.Core.Failed: Couldn't accept the call: Unhandled response '+CRING: VOICE

  +CRING: VOICE''

plugins/simtech/mm-modem-helpers-simtech.h

@@ -45,4 +45,9 @@ gboolean  mm_simtech_parse_voice_call_urc     (GMatchInfo  *match_info,
                                                guint       *duration,
                                                GError     **error);
 
+/*****************************************************************************/
+/* Non-standard CRING URC helpers */
+
+GRegex *mm_simtech_get_cring_urc_regex (void);
+
 #endif  /* MM_MODEM_HELPERS_SIMTECH_H */