Julian Brost a107792d06 Add missing include to fix non-unity builds
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
          |                        ^~~
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