param-validation: make symbols hidden

param-validation: make symbols hidden

This convenience library is linked statically with the various plugins
that use it.  The symbols are currently not marked as hidden, which
causes the plugins that use it (e.g. babeltrace-plugin-ctf.so) to expose
them publically.  Sad.

Mark the symbols as hidden to avoid that.

Change-Id: If099b529f7c126f5defd5242486009c3fd4195eb
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/2409
Tested-by: jenkins <jenkins@lttng.org>