[rygel] build: Fix building against tracker 0.18

[Jens Georg <jensgeorg src gnome org>]

commit c886acab71c54a7816a85d765936746bf1f203c4
Author: Frédéric Péters <fpeters 0d be>
Date:   Sat Dec 21 14:10:49 2013 +0100

    build: Fix building against tracker 0.18

 configure.ac |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/configure.ac b/configure.ac
index 42929df..b5b8652 100644
--- a/configure.ac
+++ b/configure.ac
@@ -325,7 +325,7 @@ AS_IF([test "x$enable_tracker_plugin" = "xyes"],
             PKG_CHECK_MODULES(TRACKER_SPARQL,
                               tracker-sparql-0.16 >=  $TRACKER_SPARQL_REQUIRED_MIN,
                               [tracker_api_version="0.16"],
-                              [])
+                              [tracker_api_version=""])
           ])
     AS_IF([test "x$[]tracker_api_version" = "x"],
           [