modem-manager: update dependencies

modem-manager: update dependencies

These are the changes implemented:

  * Remove dbus-glib as a dependency, as it isn't required since
    ModemManager 1.0, which is based on libglib2's GDBus
    implementation.

  * Remove udev as a dependency, as the ModemManager build only needs
    the libgudev package.

  * Explicitly set libglib2 as dependency, which currently was being
    implicitly included by libgudev. The next major ModemManager
    release will have udev/libgudev as optional packages, while
    libglib2 is definitely not going to be ever optional.

  * Add a comment explaining that dbus is only a runtime dependency.

Signed-off-by: Aleksander Morgado <aleksander@aleksander.es>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>