package/iwd: add dbus compile time dependency

In 5b3b2d80 we dropped dbus as a build
dependency, however we still need it when building with systemd so
that the service directory is available via pkg-config.

In addition we can drop --with-dbus-datadir by unconditionally
requiring dbus as the datadir will then be fetched from pkg-config.

Fixes:
checking D-Bus bus services directory... configure: error: D-Bus bus services directory is required

  http://autobuild.buildroot.net/results/4a48676460e6ce588897598f0022ec840b4b4b8d/



Signed-off-by: James Hilliard <james.hilliard1@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>