ANDROID: gki_defconfig: enable IPV6_MROUTE

Enable support for IPv6 multicast forwarding, required for devices with
border routing capabilities. Note this option will likely be unset after
making ipv6 a GKI module.

Bug: 233821827
Signed-off-by: Carlos Llamas <cmllamas@google.com>
Change-Id: I0ffabbe21373ac23bc353abda51862eba0135972