GCC 4.5 compatibility: can't default destructor in class body.

src/AbstractImage.h

@@ -81,7 +81,7 @@ class MAGNUM_EXPORT AbstractImage {
         std::size_t pixelSize() const { return pixelSize(_format, _type); }
 
     protected:
-        ~AbstractImage() = default;
+        ~AbstractImage();
 
     #ifdef DOXYGEN_GENERATING_OUTPUT
     private:
@@ -92,6 +92,9 @@ class MAGNUM_EXPORT AbstractImage {
         ImageType _type;
 };
 
+/* GCC 4.5 doesn't take `= default` as inline */
+inline AbstractImage::~AbstractImage() = default;
+
 }
 
 #endif