Math: proper forward declaration for Constants.

Messed that up in c5d31d74e3, causing lots
of Clang warnings. Sorry 'bout that.

src/Magnum/Math/Math.h

@@ -41,7 +41,7 @@ namespace Magnum { namespace Math {
 #ifndef DOXYGEN_GENERATING_OUTPUT
 template<std::size_t> class BoolVector;
 
-template<class> class Constants;
+template<class> struct Constants;
 
 template<class> class Complex;
 template<class> class Dual;