ALL: Require DECLARE_SINGLETON to be used in the Common namepsace

Silences the clang warning:

  static data member specialization of '_singleton' must
  originally be declared in namespace 'Common'; accepted as a C++0x
  extension [-Wc++0x-extensions]

Wrapping "namespace Common {}" around the macro assignment causes clang
to complain about a spurious semicolon, and removing the semicolon at
the end of the macro causes some editors to misbehave.

Changing the requirement of using the macro in one namespace (the
global) to another (Common) seems a small price to pay to
silence a warning.

common/debug.cpp

@@ -29,10 +29,10 @@
 // TODO: Move gDebugLevel into namespace Common.
 int gDebugLevel = -1;
 
-DECLARE_SINGLETON(Common::DebugManager);
-
 namespace Common {
 
+DECLARE_SINGLETON(DebugManager);
+
 namespace {
 
 struct DebugLevelComperator {