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

Since commit "c53eeaf75a04 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>

configure

@@ -1271,7 +1271,7 @@ for config in $mak_wilds; do
 done
 
 # Enumerate public trace backends for --help output
-trace_backend_list=$(echo $(grep -le '^PUBLIC = True$' scripts/tracetool/backend/*.py | sed -e 's/^.*\/\(.*\)\.py$/\1/'))
+trace_backend_list=$(echo $(grep -le '^PUBLIC = True$' "$source_path"/scripts/tracetool/backend/*.py | sed -e 's/^.*\/\(.*\)\.py$/\1/'))
 
 if test x"$show_help" = x"yes" ; then
 cat << EOF