net: rmnet: add missing module alias

In the current rmnet code, there is no module alias.
So, RTNL couldn't load rmnet module automatically.

Test commands:
    ip link add dummy0 type dummy
    modprobe -rv rmnet
    ip link add rmnet0 link dummy0 type rmnet  mux_id 1

Signed-off-by: Taehee Yoo <ap420073@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>