ARM: exynos_defconfig: Enable IP multicast

Prior to Nikolay's commit dcd87999, libnss-mdns (eg. to resolve
"foo.local" on the LAN using mDNS) worked without CONFIG_IP_MULTICAST
defined, but dcd87999 caused it to stop working without having
CONFIG_IP_MULTICAST defined explicitly.

Since exynos_defconfig did not previously define CONFIG_IP_MULTICAST,
mDNS used to work for users building with just exynos_defconfig, but
stopped working from dcd87999.

Define CONFIG_IP_MULTICAST explicitly in exynos_defconfig as it appears
essential anyway.

Signed-off-by: Robie Basak <robie@justgohome.co.uk>
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>