configure: fix trace backend list for out-of-tree builds

Since commit "c53eeaf7 configure: eliminate Python dependency for
--help", configure --help fails to produce the list of available trace
backends if invoked out-of-tree. It also spits the following error:

grep: scripts/tracetool/backend/*.py: No such file or directory

This patch simply adds the missing $source_path to fix it.

Signed-off-by: Greg Kurz <groug@kaod.org>
Message-id: 149321376763.7874.12797658801011614451.stgit@bahia
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>