Move perf counter symbols to private header

This is an API change but the major SONAME bump to '1' gives us the
opportunity to hide symbols that should have been private from the
start.

This also allows to stop exposing LTTNG_UST_HAVE_PERF_EVENT in the
public headers.

Change-Id: I43893b7bad6d447e86af33ad99def6c76689123f
Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>