bad libpng16 link in build script

[kostas995]

The source address is unreachable, should be changed
"https://sourceforge.net/projects/libpng/files/libpng16/$CURRENT_PACKAGE_VERSION/libpng-$CURRENT_PACKAGE_VERSION.tar.gz"

[hobodrifterdavid]

I had this problem, updated the address in the build-ffmpeg, works. :)