selftests: forwarding: gre_multipath: Enable IPv4 forwarding

The test did not enable IPv4 forwarding during its setup phase, which
causes the test to fail on machines where IPv4 forwarding is disabled.

Fixes: 54818c4c4b93 ("selftests: forwarding: Test multipath tunneling")
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Reported-by: Stephen Suryaputra <ssuryaextr@gmail.com>
Tested-by: Stephen Suryaputra <ssuryaextr@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>