Enable gofmt preupload check

All existing .go files are compatible with gofmt -w.

Test: attempt to upload badly formatted file.
Change-Id: I027935a5a5d28dcfc84ed0fbb9e0ed0beb01cfa2
diff --git a/PREUPLOAD.cfg b/PREUPLOAD.cfg
index 304c3c1..6ab01dc 100644
--- a/PREUPLOAD.cfg
+++ b/PREUPLOAD.cfg
@@ -4,6 +4,7 @@
 [Builtin Hooks]
 cpplint = true
 bpfmt = true
+gofmt = true
 
 [Builtin Hooks Options]
 # Cpplint prints nothing unless there were errors.