gnome-panel: add PACKAGECONFIG for networkmanager

* fixes:
  WARN: gnome-panel: gnome-panel rdepends on dbus-lib, but it isn't a build dependency?
* there is general dbus dependency already, always used by libpanel-applet package:
  gnome-panel.full/temp/log.do_package:DEBUG: libpanel-applet: Dependency libdbus-1.so.3 requires package dbus-lib (used by files: /home/jenkins/oe/world/shr-core/tmp-glibc/work/i586-oe-linux/gnome-panel/2.32.1-r7/packages-split/libpanel-applet/usr/lib/libpanel-applet-3.so.0.0.6)

* but when networkmanager is detected, then it's also linked from
  clock-applet in gnome-panel package:
  gnome-panel.full/temp/log.do_package:DEBUG: gnome-panel: Dependency libdbus-1.so.3 requires package dbus-lib (used by files: /home/jenkins/oe/world/shr-core/tmp-glibc/work/i586-oe-linux/gnome-panel/2.32.1-r7/packages-split/gnome-panel/usr/libexec/clock-applet)

* OE qemux86@ ~/oe/world/shr-core/tmp-glibc/work/i586-oe-linux/gnome-panel/2.32.1-r7/git $ git grep HAVE_NETWORK_MANAGER .
  applets/clock/clock-location.c:#ifdef HAVE_NETWORK_MANAGER
  applets/clock/clock-location.c:#ifdef HAVE_NETWORK_MANAGER
  applets/clock/clock-location.c:#ifdef HAVE_NETWORK_MANAGER

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>