openssl: drop libdl from pc file

Fixes:
http://autobuild.buildroot.net/results/2a2/2a274e2dbffd268a391b0e8a15dae5a759b870a1/



[Thomas: move registration of the hook inside the condition.]

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>