libsvg: fix undefined symbol png_set_gray_1_2_4_to_8

Fixes the following runtime error e.g. from df_texture:

(!) Direct/Interface: Unable to dlopen `/usr/lib/directfb-1.6-0/interfaces/IDirectFBImageProvider/libidirectfbimageprovider_svg.so'!
    --> /usr/lib/libsvg.so.1: undefined symbol: png_set_gray_1_2_4_to_8

Signed-off-by: Peter Seiderer <ps.report@gmx.net>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>