package/civetweb: fix linking failure caused by wrong argument passed to pkg-config

package/civetweb: fix linking failure caused by wrong argument passed to pkg-config

On commit 027a8b29f1e62d5ff5bbb15b79376614f902a680 pkg-config has been
added to retrieve OpenSSL dependencies, but it's been passed `libssl`
instead of `openssl`, this makes fail some linking. Indeed we need
OpenSSL dependency, so let's use `openssl` with pkg-config.

Substitute `libssl` with `openssl`.

Fixes:

  http://autobuild.buildroot.net/results/b225425ee237852bd9fee4ca0b8d24f3e37d64f9/

Signed-off-by: Giulio Benetti <giulio.benetti@micronovasrl.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>