ART: Add missing namespace comments

Add closing namespace comments.

Bug: 32619234
Test: m
Change-Id: I1f50e09dcd1038c4b540b87e5c19e319c1f592e4
diff --git a/compiler/utils/label.h b/compiler/utils/label.h
index 0f82ad5..4c6ae8e 100644
--- a/compiler/utils/label.h
+++ b/compiler/utils/label.h
@@ -29,24 +29,24 @@
 namespace arm {
   class ArmAssembler;
   class Thumb2Assembler;
-}
+}  // namespace arm
 namespace arm64 {
   class Arm64Assembler;
-}
+}  // namespace arm64
 namespace mips {
   class MipsAssembler;
-}
+}  // namespace mips
 namespace mips64 {
   class Mips64Assembler;
-}
+}  // namespace mips64
 namespace x86 {
   class X86Assembler;
   class NearLabel;
-}
+}  // namespace x86
 namespace x86_64 {
   class X86_64Assembler;
   class NearLabel;
-}
+}  // namespace x86_64
 
 class ExternalLabel {
  public: