libpng: Use working pkg url

packages/libpng/build.sh

@@ -2,5 +2,6 @@ TERMUX_PKG_HOMEPAGE=http://www.libpng.org/pub/png/libpng.html
 TERMUX_PKG_DESCRIPTION="Official PNG reference library"
 TERMUX_PKG_VERSION=1.6.18
 TERMUX_PKG_BUILD_REVISION=1
-TERMUX_PKG_SRCURL=ftp://ftp.simplesystems.org/pub/libpng/png/src/libpng16/libpng-${TERMUX_PKG_VERSION}.tar.xz
+TERMUX_PKG_SRCURL=http://downloads.sourceforge.net/project/libpng/libpng16/${TERMUX_PKG_VERSION}/libpng-${TERMUX_PKG_VERSION}.tar.xz
+# ftp://ftp.simplesystems.org/pub/libpng/png/src/libpng16/libpng-${TERMUX_PKG_VERSION}.tar.xz
 TERMUX_PKG_RM_AFTER_INSTALL="bin/libpng-config bin/libpng16-config bin/png-fix-itxt bin/pngfix"