Fix errors caught by go vet in compliance package

`go test` implicitly runs `go vet` and fails the tests if vet
errors are found.  Fix all the issues found by vet.

Test: go test build/make/tools/compliance/...
Change-Id: If0684cf124ece4931af440008cd44a61c22de475
diff --git a/tools/compliance/policy/policy_test.go b/tools/compliance/policy/policy_test.go
index 09e831c..32dd5fd 100644
--- a/tools/compliance/policy/policy_test.go
+++ b/tools/compliance/policy/policy_test.go
@@ -226,7 +226,7 @@
 			fs[tt.edge.dep] = []byte(meta[tt.edge.dep])
 			lg, err := ReadLicenseGraph(&fs, stderr, []string{tt.edge.target})
 			if err != nil {
-				t.Errorf("unexpected error reading graph: %w", err)
+				t.Errorf("unexpected error reading graph: %s", err)
 				return
 			}
 			edge := lg.Edges()[0]