package/nginx: fix libxslt support

Change to using pkg-config to find libxslt and it dependencies.

This Fixes:
 http://autobuild.buildroot.net/results/382699bbed15f598625b9296e464d1349ef559f8



Signed-off-by: Martin Bark <martin@barkynet.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>