plugins: generate dummy main function for plugins to fix go#20312

https://github.com/golang/go/issues/20312

License: MIT
Signed-off-by: Jakub Sztandera <kubuxu@protonmail.ch>

plugin/plugins/gen_main.sh

@@ -15,4 +15,8 @@ import (
 )
 
 var Plugins = $shortpkg.Plugins
+
+func main() {
+	panic("this is a plugin, build it as a plugin, this is here as for go#20312")
+}
 EOL