package/polkit: drop dbus build dependency

Drop dbus build dependency to avoid the following build failure since
commit 1db13226 if systemd-polkit is
enabled:

package/dbus/dbus.mk:124: *** Recursive variable 'DBUS_FINAL_RECURSIVE_DEPENDENCIES' references itself (eventually).  Stop.

Fixes:
 - http://autobuild.buildroot.org/results/0e038fae0f5fc2db3e85be05db4612e4f2395e35



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>