Math: provide forward declaration for Constants.

No point in having it repeated in Magnum.h, since we include Math/Math.h
anyway.

src/Magnum/Magnum.h

@@ -48,12 +48,6 @@ typedef unsigned int GLenum; /* Needed for *Format and *Type enums */
 
 namespace Magnum {
 
-namespace Math {
-    #ifndef DOXYGEN_GENERATING_OUTPUT
-    template<class> struct Constants;
-    #endif
-}
-
 /* Bring whole Corrade namespace */
 using namespace Corrade;
 

src/Magnum/Math/Math.h

@@ -41,7 +41,7 @@ namespace Magnum { namespace Math {
 #ifndef DOXYGEN_GENERATING_OUTPUT
 template<std::size_t> class BoolVector;
 
-/* Class Constants used only statically */
+template<class> class Constants;
 
 template<class> class Complex;
 template<class> class Dual;