Math: forgot to add Complex to forward-declaration header.

src/Math/Math.h

@@ -27,6 +27,7 @@ namespace Magnum { namespace Math {
 #ifndef DOXYGEN_GENERATING_OUTPUT
 /* Class Constants used only statically */
 
+template<class T> class Complex;
 template<class T> class Dual;
 template<class> class DualQuaternion;