Math: fixed test compilation with libc++.

Apparently std::forward() is constexpr only in C++14. Heh.

src/Magnum/Math/Test/VectorTest.cpp

@@ -449,7 +449,7 @@ void VectorTest::angle() {
 
 template<class T> class BasicVec2: public Math::Vector<2, T> {
     public:
-        template<class ...U> constexpr BasicVec2(U&&... args): Math::Vector<2, T>{std::forward<U>(args)...} {}
+        template<class ...U> constexpr BasicVec2(U&&... args): Math::Vector<2, T>{args...} {}
 
         MAGNUM_VECTOR_SUBCLASS_IMPLEMENTATION(2, BasicVec2)
 };