Add missing include to fix non-unity builds

[julianbrost]

This commit fixes the following build error:

[ 55%] Building CXX object lib/icinga/CMakeFiles/icinga.dir/usergroup.cpp.o
lib/icinga/usergroup.cpp:79:24: error: incomplete type ‘icinga::Notification’ used in nested name specifier
   79 | std::set<Notification::Ptr> UserGroup::GetNotifications() const
      |                        ^~~

Note that this is also broken in 2.13.2 (#9009 (comment)) so the fix should also be released in 2.13.3.

[Al2Klimov]

Note that this is also broken in 2.13.2 (#9009 (comment)) so the fix should also be released in 2.13.3.

What for? Where do we have non-unity builds?

[julianbrost]

We don't, otherwise we would have noticed ourselves. It should work nonetheless. But nothing for which we'd need an immediate release or something like that.