Make err_data_generate.go reproducible.

Sort all the files before processing them.

Change-Id: Id6b4519fa22f1770bb2ba2a792f5c27de9ea452d
Reviewed-on: https://boringssl-review.googlesource.com/3380
Reviewed-by: Adam Langley <agl@google.com>
diff --git a/crypto/err/err_data_generate.go b/crypto/err/err_data_generate.go
index 925b45e..003f7f4 100644
--- a/crypto/err/err_data_generate.go
+++ b/crypto/err/err_data_generate.go
@@ -238,6 +238,7 @@
 		panic(err)
 	}
 
+	sort.Strings(names)
 	for _, name := range names {
 		if !strings.HasSuffix(name, ".errordata") {
 			continue