In core ("src/"), we use "nm-logging.h" for all logging. This dispatches
for logging to syslog, glog or systemd-journald.

If we want to log from a shared component under "shared/", we need to
use a common logging function. Add "nm-utils/nm-logging-fwd.h" for
forward declaring the used logging mechaism.

The shared library will still need to link with "src/nm-logging.c"
or an alternative implementation, depending on whether it is used
inside core or not.