Switch to libpng16 in libgdk_pixbuf_dev

[Jeroen Ooms]

Hello,

I am trying to test R bindings for librsvg2. However I get this error:

  pkg-config --cflags librsvg-2.0
  Package libpng15 was not found in the pkg-config search path.
  Perhaps you should add the directory containing `libpng15.pc'
  Package 'libpng15', required by 'GdkPixbuf', not found

The problem is that gdk-pixbuf-2.0.pc contains this line:

    Requires.private: gmodule-no-export-2.0 libpng15

However I think libpng15.pc no longer exists? The solution is simply
to change it to libpng16. I don't think this would require a rebuild.

Thank you,

Jeroen