Windows build fixes.

src/AbstractImage.h

@@ -28,7 +28,7 @@ namespace Magnum {
 
 See Image, BufferedImage, Trade::ImageData documentation for more information.
 */
-class AbstractImage {
+class MAGNUM_EXPORT AbstractImage {
     AbstractImage(const AbstractImage& other) = delete;
     AbstractImage(AbstractImage&& other) = delete;
     AbstractImage& operator=(const AbstractImage& other) = delete;

src/Renderbuffer.h

@@ -83,7 +83,7 @@ class Renderbuffer {
         };
 
         /** @copydoc AbstractTexture::InternalFormat */
-        class InternalFormat {
+        class MAGNUM_EXPORT InternalFormat {
             public:
                 /** @copydoc AbstractTexture::InternalFormat::InternalFormat(AbstractTexture::Components, AbstractTexture::ComponentType) */
                 InternalFormat(Components components, ComponentType type);