tools: fix building failures on Windows

When building tools on Win,it shows `.\main.go:68:12: assignment mismatch: 2 variables but 1 values`.The reason is the return variables not match the calling from `main.go` and i try to fix it.

tools/etcd-dump-metrics/install_windows.go

@@ -18,6 +18,6 @@ package main
 
 import "errors"
 
-func install(ver, dir string) error {
-	return errors.New("windows install is not supported yet")
+func install(ver, dir string) (string, error) {
+	return "", errors.New("windows install is not supported yet")
 }