kernel-wfp: Set flag to get UDP encapsulation with tunnel mode working

Having this flag set fixes connections initiated by the Windows host, but
unfortunately does not yet fix incoming connections. Connection state issue?
We still see 0xc00000e2 error events, translating to INTERNAL_ERROR.

src/libcharon/plugins/kernel_wfp/kernel_wfp_ipsec.c

@@ -913,6 +913,7 @@ static bool install_sa(private_kernel_wfp_ipsec_t *this, entry_t *entry,
 	}
 	else
 	{
+		bundle.flags |= IPSEC_SA_BUNDLE_FLAG_ASSUME_UDP_CONTEXT_OUTBOUND;
 		res = IPsecSaContextAddOutbound0(this->handle, entry->sa_id, &bundle);
 	}
 	if (res != ERROR_SUCCESS)