slot: shorten slot type

Create an alias `sdbusplus::slot_t` to `sdbusplus::slot::slot`
to reduce duplication.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I0ac45f676f1e52f53b6ea3a5b487d0f183641e37
diff --git a/include/sdbusplus/slot.hpp b/include/sdbusplus/slot.hpp
index 27f4a70..5fa9141 100644
--- a/include/sdbusplus/slot.hpp
+++ b/include/sdbusplus/slot.hpp
@@ -73,4 +73,7 @@
 };
 
 } // namespace slot
+
+using slot_t = slot::slot;
+
 } // namespace sdbusplus