[codegen] fix unused fmt bug (#1118)

## What Changed? Why?
Sometimes fmt is not used, this checks the only condition it would be
used.

### How was it tested?
Locally

Author: amalavet

codegen/templates/schema.tmpl

@@ -4,10 +4,11 @@
 
 package {{.Package}}
 {{$sfl := len .SelectableFields}}
+{{$needsFmt := false}}{{range .SelectableFields}}{{if ne .Type "string"}}{{$needsFmt = true}}{{end}}{{end}}
 import (
     {{if gt $sfl 0}}
-    "fmt"
-    "errors"
+    "errors"{{if $needsFmt}}
+    "fmt"{{end}}
     {{end}}
 
     "github.com/grafana/grafana-app-sdk/resource"