TT#61954 Set debhelper compat level also for ngcp-rtpengine-kernel-source package

This was forgotten to be taken care of in commit 2868fc6f55.

Otherwise `module-assistant auto-install ngcp-rtpengine-kernel-source` fails with:

| dh_installdocs: Please specify the compatibility level in debian/compat
| make[1]: *** [debian/rules:64: binary-modules] Error 2
| make[1]: Leaving directory '/usr/src/modules/ngcp-rtpengine'

Fixes #853

Change-Id: I0a433daaaebdab09f8247a391624d8f1a6d045bc

debian/control.modules.in

@@ -3,7 +3,7 @@ Section: kernel
 Priority: optional
 Maintainer: Sipwise Development Team <support@sipwise.com>
 Build-Depends:
- debhelper (>= 10),
+ debhelper-compat (= 12),
 Standards-Version: 3.9.8
 Homepage: https://www.sipwise.com/