package/htpdate: fix build with static openssl

package/htpdate: fix build with static openssl

Depending on its build configuration, openssl might not be compiled with
dynamic zlib support, and programs that link against openssl need to link
against zlib as well.

This can be generally and easily handled using the pkg-config files
installed by openssl.

Fixes: http://autobuild.buildroot.net/results/ae9/ae946ca72238840b3eaa5fe823e8d620618f7462/
Signed-off-by: Titouan Christophe <titouan.christophe@railnova.eu>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>