selftests/bpf: Enable generic tc actions in selftests config

selftests/bpf: Enable generic tc actions in selftests config

Enable CONFIG_NET_ACT_GACT to allow adding simple actions with tc
filters. This is for example needed to migrate test_flow_dissector into
the automated testing performed in CI.

Acked-by: Stanislav Fomichev <sdf@fomichev.me>
Signed-off-by: Alexis Lothoré (eBPF Foundation) <alexis.lothore@bootlin.com>
Link: https://lore.kernel.org/r/20241120-flow_dissector-v3-8-45b46494f937@bootlin.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>