follow golang/go#25922 for dev tools

main.go

@@ -1,4 +1,4 @@
-//go:generate statik -src=web/ -include=*.html,*.css,*.js,*.template
+//go:generate statik -f -src=web/ -include=*.html,*.css,*.js,*.template
 
 package main
 

tools.go

@@ -0,0 +1,7 @@
+// +build tools
+
+package tools
+
+import (
+        _ "github.com/rakyll/statik"
+)