make vec2 operator implementation the same as with vec3

[ahmidou]

This is making inheriting from the vec2 type failing to build
for example

template<typename T, qualifier Q>
	template<typename U>
	GLM_FUNC_QUALIFIER GLM_CONSTEXPR vec<2, T, Q> & vec<2, T, Q>::operator+=(U scalar)
	{
		this->x += static_cast<T>(scalar);
		this->y += static_cast<T>(scalar);
         }

should be

	template<typename T, qualifier Q>
	template<typename U>
	GLM_FUNC_QUALIFIER GLM_CONSTEXPR vec<2, T, Q> & vec<2, T, Q>::operator+=(U scalar)
	{
		return (*this = detail::compute_vec_add<2, T, Q, detail::is_aligned<Q>::value>::call(*this, vec<2, T, Q>(scalar)));
	{