package/dbus: fix group and mode for the launcher

package/dbus: fix group and mode for the launcher

The D-Bus installation process installs dbus-daemon-launch-helper as
follows:

  chown root:$(DBUS_USER) $(DESTDIR)$(libexecdir)/dbus-daemon-launch-helper$(EXEEXT); \
  chmod 4750 $(DESTDIR)$(libexecdir)/dbus-daemon-launch-helper$(EXEEXT); \

And when the installation does not take place as root (like is the
case in the context of Buildroot), it prints:

  echo "Not installing $(DESTDIR)$(libexecdir)/dbus-daemon-launch-helper binary setuid!"; \
  echo "You'll need to manually set permissions to root:$(DBUS_USER) and permissions 4750"; \

So let's adjust the installation logic of dbus-daemon-launch-helper to
match these requirements.

Signed-off-by: Norbert Lange <nolange79@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>