fetch zlib from the cannonical site at zlib.net

package/zlib/zlib.mk

@@ -5,7 +5,7 @@
 #############################################################
 ZLIB_VER:=1.2.3
 ZLIB_SOURCE:=zlib-$(ZLIB_VER).tar.bz2
-ZLIB_SITE:=http://$(BR2_SOURCEFORGE_MIRROR).dl.sourceforge.net/sourceforge/libpng
+ZLIB_SITE:=http://www.zlib.net/
 ZLIB_DIR:=$(BUILD_DIR)/zlib-$(ZLIB_VER)
 ZLIB_CFLAGS:=$(TARGET_CFLAGS) -fPIC
 ifeq ($(BR2_LARGEFILE),y)