fix libpng download url

Author: koflin

build-ffmpeg

@@ -817,7 +817,7 @@ if build "libtiff" "4.5.0"; then
   build_done "libtiff" $CURRENT_PACKAGE_VERSION
 fi
 if build "libpng" "1.6.39"; then
-  download "https://gigenet.dl.sourceforge.net/project/libpng/libpng16/$CURRENT_PACKAGE_VERSION/libpng-$CURRENT_PACKAGE_VERSION.tar.gz" "libpng-$CURRENT_PACKAGE_VERSION.tar.gz"
+  download "https://sourceforge.net/projects/libpng/files/libpng16/$CURRENT_PACKAGE_VERSION/libpng-$CURRENT_PACKAGE_VERSION.tar.gz" "libpng-$CURRENT_PACKAGE_VERSION.tar.gz"
   export LDFLAGS="${LDFLAGS}"
   export CPPFLAGS="${CFLAGS}"
   execute ./configure --prefix="${WORKSPACE}" --disable-shared --enable-static