Removed unneeded `extern class`, added export macros to proper place.

Leftover from c4f321d911, don't know at
all why and how that happen.

src/BufferImage.cpp

@@ -34,9 +34,9 @@ template<UnsignedInt dimensions> void BufferImage<dimensions>::setData(const typ
     _buffer.setData(pixelSize(format, type)*size.product(), data, usage);
 }
 
-template class BufferImage<1>;
-template class BufferImage<2>;
-template class BufferImage<3>;
+template class MAGNUM_EXPORT BufferImage<1>;
+template class MAGNUM_EXPORT BufferImage<2>;
+template class MAGNUM_EXPORT BufferImage<3>;
 #endif
 
 }

src/Image.cpp

@@ -34,8 +34,8 @@ template<UnsignedInt dimensions> void Image<dimensions>::setData(const typename
     _data = reinterpret_cast<unsigned char*>(data);
 }
 
-template class Image<1>;
-template class Image<2>;
-template class Image<3>;
+template class MAGNUM_EXPORT Image<1>;
+template class MAGNUM_EXPORT Image<2>;
+template class MAGNUM_EXPORT Image<3>;
 
 }

src/Image.h

@@ -94,12 +94,6 @@ template<UnsignedInt dimensions> class Image: public AbstractImage {
         unsigned char* _data;
 };
 
-#ifndef DOXYGEN_GENERATING_OUTPUT
-extern template class MAGNUM_EXPORT Image<1>;
-extern template class MAGNUM_EXPORT Image<2>;
-extern template class MAGNUM_EXPORT Image<3>;
-#endif
-
 /** @brief One-dimensional image */
 typedef Image<1> Image1D;