MT#61302 Ensure to have fence-agents-common available on trixie

In Debian trixie, the fence-agents package is an empty package which has
all its actual fence-agents in Recommends only since
0ac9bfcc4b
As of
8cc112f623
the fence-agents source package provides a fence-agents-common binary
package.

If we install only the fence-agents package with Recommends disabled,
we end up without any actual fence agent, and also missing file
/usr/share/fence/fencing.py shipped by fence-agents-common.
Therfore our fence_ngcp fails hard:

| root@sp1:~# /usr/sbin/fence_ngcp
| Traceback (most recent call last):
|   File "/usr/sbin/fence_ngcp", line 11, in <module>
|     from fencing import *  # noqa: E402
|     ^^^^^^^^^^^^^^^^^^^^^
| ModuleNotFoundError: No module named 'fencing'

Reported as https://bugs.debian.org/1085256

NOTE: applied change from system-toools.git, see
commit dc2f41e074b3a91ac3174dd65c370a856d576a34

Change-Id: I8471f264a7f2a398a45fdaeefda8bdd3435f5c4f

debian/control

@@ -296,7 +296,7 @@ Depends:
  dbus,
  ethtool,
  exim4,
- fence-agents,
+ fence-agents-common | fence-agents,
  ffproxy,
  freeswitch,
  freeswitch-mod-callcenter,