vtable: shorten vtable type

Create an alias `sdbusplus::vtable_t` to `sdbusplus::vtable::vtable_t`
to reduce duplication.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I0c3a8e40827cb011a07d75c99550a371afdefc64
diff --git a/test/vtable/vtable.cpp b/test/vtable/vtable.cpp
index 9b563a6..32f72a8 100644
--- a/test/vtable/vtable.cpp
+++ b/test/vtable/vtable.cpp
@@ -17,7 +17,7 @@
     extern const size_t example2_size;
 }
 
-static const sdbusplus::vtable::vtable_t example[] = {
+static const sdbusplus::vtable_t example[] = {
     sdbusplus::vtable::start(),
     sdbusplus::vtable::method("1", "2", "3", &test_handler, 0),
     sdbusplus::vtable::signal("5", "6"),