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.cpp b/test/vtable/vtable.cpp
index 5ed38b8..abb27d6 100644
--- a/test/vtable/vtable.cpp
+++ b/test/vtable/vtable.cpp
@@ -24,7 +24,7 @@
     sdbusplus::vtable::property("7", "8", &test_get,
                                 sdbusplus::vtable::property_::const_),
     sdbusplus::vtable::property("10", "11", &test_get, &test_set),
-    sdbusplus::vtable::property_o("14", "15", 16),
+    sdbusplus::vtable::property_o("14", "15", &test_get, 16),
     sdbusplus::vtable::end()};
 
 TEST(VtableTest, SameSize)