dbus: use ${PN} in pkg_postinst instead of 'dbus'

dbus: use ${PN} in pkg_postinst instead of 'dbus'

All pkg_postinst in oe-core where the package name matches the recipe
name use ${PN} already.

We have a bbclass used by some recipes which does:
pkg_postinst:${PN}:append()
which works reasonably well for most of the recipes, except for dbus
where it causes:

WARNING: meta/recipes-core/dbus/dbus_1.16.2.bb:
  Variable key pkg_postinst:${PN} (...)
  replaces original key pkg_postinst:dbus (...)

Signed-off-by: Martin Jansa <martin.jansa@gmail.com>
(cherry picked from commit efec0447c8e8a6003f00642b33a71ed94fc4ec82)
Signed-off-by: Yoann Congal <yoann.congal@smile.fr>