provider/google: Hook in state migration function

As part of Terraform 0.7.1 it was observed in issue #8345 that the state
migration for google_compute_firewall did not appear to be running,
causing a panic when an uninitialized member was read. This commit hooks
up the state migration function (which _was_ independently unit tested
but was not actually in place).

There is currently no good test framework for this, I will address this
issue in a future RFC.

builtin/providers/google/resource_compute_firewall.go

@@ -23,6 +23,7 @@ func resourceComputeFirewall() *schema.Resource {
 			State: schema.ImportStatePassthrough,
 		},
 		SchemaVersion: 1,
+		MigrateState:  resourceComputeFirewallMigrateState,
 
 		Schema: map[string]*schema.Schema{
 			"name": &schema.Schema{