Add spaces around SRC_URI in dbus-service class

Without this, subsequent prepend/appends to SRC_URI don't work.
diff --git a/meta-phosphor/classes/obmc-phosphor-dbus-service.bbclass b/meta-phosphor/classes/obmc-phosphor-dbus-service.bbclass
index 4aacca4..1b175ea 100644
--- a/meta-phosphor/classes/obmc-phosphor-dbus-service.bbclass
+++ b/meta-phosphor/classes/obmc-phosphor-dbus-service.bbclass
@@ -6,7 +6,7 @@
         services = d.getVar('DBUS_SERVICES', True)
         if services:
                 uris = " ".join( [ 'file://' + s + '.conf' for s in services.split() ] )
-                d.appendVar('SRC_URI', uris)
+                d.appendVar('SRC_URI', ' ' + uris + ' ')
 }
 
 do_install_append() {