Don't add $(EX_LIBS) to libssl.pc's Libs.private

Since libssl requires libcrypto and libcrypto.pc already has
Libs.private set exactly the same, there's no reason to repeat it in
libssl.pc.

Reviewed-by: Bernd Edlinger <bernd.edlinger@hotmail.de>
(Merged from https://github.com/openssl/openssl/pull/5135)