[clang-tidy] allow 2 empty lines

This is in line with the clang-format specs, otherwise we will loose the
2 empty lines around classes, etc...

.clang-tidy

@@ -3,7 +3,7 @@ HeaderFilterRegex: ''
 AnalyzeTemporaryDtors: false
 CheckOptions:    
   - key:             google-readability-braces-around-statements.ShortStatementLines
-    value:           '1'
+    value:           '2'
   - key:             google-readability-function-size.StatementThreshold
     value:           '800'
   - key:             google-readability-namespace-comments.ShortNamespaceLines