COMMON: Change way the Singleton instances are instantiated

This fixes tons of warnings with clang from a recent xcode version on
macOS (and possibly other systems) complaining that an instantiation
of _singleton is required but no definition is available.

graphics/fontman.cpp

@@ -26,10 +26,6 @@
 
 #include "common/translation.h"
 
-namespace Common {
-DECLARE_SINGLETON(Graphics::FontManager);
-}
-
 namespace Graphics {
 
 FORWARD_DECLARE_FONT(g_sysfont);