openldap: fix static link with openssl

Use pkg-config to get openssl link dependencies.

Fixes:
http://autobuild.buildroot.net/results/dd3/dd3821e713b7da7cde0f059409fba46371c8bc40/
http://autobuild.buildroot.net/results/d60/d60c8b4522e99cf385ee41aa20a1d49deb59c5fb/
http://autobuild.buildroot.net/results/b41/b41c043fb3b2fad1d9cea0a95b512fb4942b5b19/



and more.

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>