package/mrouted: ensure multicast routing is enabled in kernel

package/mrouted: ensure multicast routing is enabled in kernel

IP_MULTICAST depends on CONFIG_NET and CONFIG_INET, but those are really
depp-core symbols, and anyone enabling mrouted will be expected to
already have networking and TCP/IP supports enabled already in their
kernel configuraiton.

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>
[yann.morin.1998@free.fr:
  - add comment about NET and INET,
  - update commit log accordingly
]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>