GRAPHICS: Fix BMP getPalette function definition.

The BMP decoder getPalette function definition is now identical to
the other image format decoders subclassed from ImageDecoder.
This also fixes a overloaded virtual warning reported by salty-horse.

Author: digitall

graphics/decoders/bmp.h

@@ -51,7 +51,7 @@ class BitmapDecoder : public ImageDecoder {
 	void destroy();
 	virtual bool loadStream(Common::SeekableReadStream &stream);
 	virtual const Surface *getSurface() const { return _surface; }
-	virtual const byte *getPalette() { return _palette; }
+	const byte *getPalette() const { return _palette; }
 
 private:
 	Surface *_surface;