trace, lttng: require .pc files

The next version of lttng-libs will not require liburcu at run time anymore.
Therefore, it is expected that distros will not include the urcubp libraries
anymore when installing lttng-ust-devel.

To avoid future problems, just require pkg-config to detect lttng-ust.
The .pc files for lttng-ust correctly include liburcubp.a for static
builds, and have always done since pkg-config files were added in 2011.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-id: 20210712155710.520889-1-pbonzini@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>