GPRAHICS: Slight cleanup in png.cpp.

This adds an explanation why we use FORBIDDEN_SYMBOL_ALLOW_ALL and removes
some trailing whitespaces.

graphics/decoders/png.cpp

@@ -19,6 +19,9 @@
  * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
  *
  */
+
+// Since we need to work with libpng here, we need to allow all symbols
+// to avoid compilation issues.
 #define FORBIDDEN_SYMBOL_ALLOW_ALL
 #include "common/scummsys.h"