Trade: add an explicitly defaulted deinlined SkinData destructor.

Apparently now that we don't use ObjectData anymore, SkinData was kinda
relying on them to include Matrix3/Matrix4. Hah.

src/Magnum/Trade/SkinData.cpp

@@ -43,6 +43,8 @@ template<UnsignedInt dimensions> SkinData<dimensions>::SkinData(DataFlags, const
 
 template<UnsignedInt dimensions> SkinData<dimensions>::SkinData(SkinData<dimensions>&&) noexcept = default;
 
+template<UnsignedInt dimensions> SkinData<dimensions>::~SkinData() = default;
+
 template<UnsignedInt dimensions> SkinData<dimensions>& SkinData<dimensions>::operator=(SkinData<dimensions>&&) noexcept = default;
 
 template<UnsignedInt dimensions> Containers::Array<UnsignedInt> SkinData<dimensions>::releaseJointData() {

src/Magnum/Trade/SkinData.h

@@ -82,6 +82,8 @@ template<UnsignedInt dimensions> class SkinData {
         /** @brief Move constructor */
         SkinData(SkinData<dimensions>&& other) noexcept;
 
+        ~SkinData();
+
         /** @brief Copying is not allowed */
         SkinData<dimensions>& operator=(const SkinData<dimensions>&) = delete;