proposal-substructure: Fix incorrect type for IKEv2 proposals

Luckily, the type is only used once when generating payloads and there it
doesn't matter because the encoding rules are the same.

Closes strongswan/strongswan#135.

src/libcharon/encoding/payloads/proposal_substructure.c

@@ -1539,7 +1539,7 @@ proposal_substructure_t *proposal_substructure_create_from_proposal_v2(
 	private_proposal_substructure_t *this;
 
 	this = (private_proposal_substructure_t*)
-							proposal_substructure_create(PLV2_SECURITY_ASSOCIATION);
+						proposal_substructure_create(PLV2_PROPOSAL_SUBSTRUCTURE);
 	set_from_proposal_v2(this, proposal);
 	set_data(this, proposal);