selftests: net: add missing config for GENEVE

selftests: net: add missing config for GENEVE

l2_tos_ttl_inherit.sh verifies the inheritance of tos and ttl
for GRETAP, VXLAN and GENEVE.
Before testing it checks if the required module is available
and if not skips the tests accordingly.
Currently only GRETAP and VXLAN are tested because the GENEVE
module is missing.

Fixes: b690842d12fd ("selftests/net: test l2 tunnel TOS/TTL inheriting")
Signed-off-by: Matthias May <matthias.may@westermo.com>
Link: https://lore.kernel.org/r/20240130101157.196006-1-matthias.may@westermo.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>