package/mender: the dbus plugin requires libglib2

package/mender: the dbus plugin requires libglib2

If libglib2 is not build before building the dbus plugin, mender fails to
compile with the following error:
Package 'gio-2.0', required by 'virtual:world', not found

 - Add a check for libglib2 in addition to dbus when enabling the dbus plugin.
 - Depend on libglib2 if both packages are selected.

Fixes:
http://autobuild.buildroot.org/results/1bc5893b88db08612059ad899c2bc3b2abb291fb

Signed-off-by: Adam Duskett <aduskett@gmail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>