message: shorten message type

Create an alias `sdbusplus::message_t` to `sdbusplus::message::message`
to reduce duplication.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: Ib400b12fe4a412c0c0c3d26a88f3fae46445cfa8
diff --git a/test/server/message_variant.cpp b/test/server/message_variant.cpp
index fef1eff..0fcde8d 100644
--- a/test/server/message_variant.cpp
+++ b/test/server/message_variant.cpp
@@ -9,7 +9,7 @@
 struct Object : public ::testing::Test
 {
     sdbusplus::bus::bus bus = sdbusplus::bus::new_default();
-    sdbusplus::message::message msg = bus.new_method_call(
+    sdbusplus::message_t msg = bus.new_method_call(
         "xyz.openbmc_project.sdbusplus.test.Object",
         "/xyz/openbmc_project/sdbusplus/test/object",
         "xyz.openbmc_project.sdbusplus.test.Object", "Unused");