Fix vtable entires for offset callbacks

property_o didn't pass through the callbacks as sd-bus expected, so
they can't be used for read-write properties.  Add the expected methods.

Change-Id: Iad7592c99a60c22c10e274517ebfcbf8cf293a42
Signed-off-by: Ed Tanous <edtanous@google.com>
diff --git a/test/vtable/vtable_c.c b/test/vtable/vtable_c.c
index 105409f..686833b 100644
--- a/test/vtable/vtable_c.c
+++ b/test/vtable/vtable_c.c
@@ -44,7 +44,7 @@
     SD_BUS_SIGNAL("5", "6", 0),
     SD_BUS_PROPERTY("7", "8", &test_get, 0, SD_BUS_VTABLE_PROPERTY_CONST),
     SD_BUS_WRITABLE_PROPERTY("10", "11", &test_get, &test_set, 0, 0),
-    SD_BUS_PROPERTY("14", "15", NULL, 16, 0),
+    SD_BUS_PROPERTY("14", "15", &test_get, 16, 0),
     SD_BUS_VTABLE_END,
 };