libnm-util: fix flags for GObject introspection (rh #1255436)

libnm-util: fix flags for GObject introspection (rh #1255436)

Without marking the enumeration as flags, the types will be "enumeration"
instead of "bitfield". And python raises ValueError exception.

Example:
from gi.repository import NMClient, NetworkManager
client = NMClient.Client.new()
for dev in client.get_devices():
    if dev.get_device_type() == NetworkManager.DeviceType.WIFI:
        for ap in dev.get_access_points():
            ap.get_rsn_flags()

raises "ValueError: invalid enum value: 648"

https://bugzilla.redhat.com/show_bug.cgi?id=1255436