contrib/check-config: check for xt_bpf

We omit xt_u32 as it's optional; since we will remove support for this
module in the future, it's simpler to check for xt_bpf, which will
become the new baseline.

Related issues:
* https://github.com/microsoft/WSL/issues/10029#issuecomment-1574440255
* https://github.com/docker/for-win/issues/13450#issuecomment-1574443139

Signed-off-by: Bjorn Neergaard <bjorn.neergaard@docker.com>

contrib/check-config.sh

@@ -220,6 +220,7 @@ check_flags \
 	VETH BRIDGE BRIDGE_NETFILTER \
 	IP_NF_FILTER IP_NF_TARGET_MASQUERADE \
 	NETFILTER_XT_MATCH_ADDRTYPE \
+	NETFILTER_XT_MATCH_BPF \
 	NETFILTER_XT_MATCH_CONNTRACK \
 	NETFILTER_XT_MATCH_IPVS \
 	NETFILTER_XT_MARK \