Fix format string issues

Fix issues caught by go vet.

Test: m checkbuild
Change-Id: Ib8d740457c15432dabe1575a6707726ddaf93084
diff --git a/cc/cc_test.go b/cc/cc_test.go
index 2efee59..437211c 100644
--- a/cc/cc_test.go
+++ b/cc/cc_test.go
@@ -227,7 +227,7 @@
 
 	mod := ctx.ModuleForTests(name, vendorVariant).Module().(*Module)
 	if !mod.hasVendorVariant() {
-		t.Error("%q must have vendor variant", name)
+		t.Errorf("%q must have vendor variant", name)
 	}
 
 	// Check library properties.