package/nginx-dav-ext: needs libxslt

Build is broken since bump to version 3.0.0 in commit
45524f10 because libxslt is needed
instead of expat since
https://github.com/arut/nginx-dav-ext-module/commit/37772c545314aa4a250066f4e7cbc8806d406fa8:

adding module in /tmp/instance-0/output-1/build/nginx-dav-ext-3.0.0
 + ngx_http_dav_ext_module was configured
checking for PCRE library ... found
checking for PCRE JIT support ... found
Package libxslt was not found in the pkg-config search path.
Perhaps you should add the directory containing `libxslt.pc'
to the PKG_CONFIG_PATH environment variable
Package 'libxslt', required by 'virtual:world', not found
Package libxslt was not found in the pkg-config search path.
Perhaps you should add the directory containing `libxslt.pc'
to the PKG_CONFIG_PATH environment variable
Package 'libxslt', required by 'virtual:world', not found
checking for libxslt ... not found
checking for libxslt in /usr/local/ ... not found
checking for libxslt in /usr/pkg/ ... not found
checking for libxslt in /opt/local/ ... not found

./configure: error: the HTTP XSLT module requires the libxml2/libxslt
libraries. You can either do not enable the module or install the libraries.

Fixes:
 - http://autobuild.buildroot.org/results/8735d20ca7ccf4eda2f79f9400bed70474855b40



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>