sdbus++: move interface name to common header

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I887b85d47e4e380426973f217bb94b1bec69f7aa
diff --git a/example/calculator-server.cpp b/example/calculator-server.cpp
index 9e02ba0..aa60662 100644
--- a/example/calculator-server.cpp
+++ b/example/calculator-server.cpp
@@ -56,7 +56,7 @@
 
     static_assert(
         std::string_view(
-            sdbusplus::client::net::poettering::Calculator::interface) ==
+            sdbusplus::common::net::poettering::Calculator::interface) ==
         std::string_view(Calculator::interface));
 
     // Create a new bus and affix an object manager for the subtree path we