cpplint: Cleanup errors

Cleanup errors from upstream cpplint in preparation
for moving art's cpplint fork to upstream tip-of-tree cpplint.

Test: cd art && mm
Bug: 68951293
Change-Id: I15faed4594cbcb8399850f8bdee39d42c0c5b956
diff --git a/runtime/stack.h b/runtime/stack.h
index 4ef9487..bd6204f 100644
--- a/runtime/stack.h
+++ b/runtime/stack.h
@@ -27,7 +27,7 @@
 namespace art {
 
 namespace mirror {
-  class Object;
+class Object;
 }  // namespace mirror
 
 class ArtMethod;