package/nginx: fix openssl support

Change to using pkg-config to find openssl and its dependencies.

This fixes:
 http://autobuild.buildroot.net/results/17db76c3a099e7592af343bbe119dfe717cd7999/



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