libxslt: update to version 1.1.28

Recently updated libxml2 from version 2.7.8 to 2.9.1 breaks
libxslt build. Library libxslt from version 1.1.27 is compatible
with libxml2 2.9.x.

Signed-off-by: Lukas Macura <lukas.macura@cesnet.cz>
Tested-by: Jiri Slachta <slachta@cesnet.cz>
Signed-off-by: Florian Fainelli <florian@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/packages@40556 3c298f89-4303-0410-b956-a3cf2f4a3e73