sdbus++: async: client: fix clang warning on struct mismatch

The mako template generates a `struct` and then later lists it as
a `friend class`.  clang warns about this mismatch.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I004b44df6b1a5ac97b4a51aeb2d13e78ac0cf31e
diff --git a/tools/sdbusplus/templates/interface.client.hpp.mako b/tools/sdbusplus/templates/interface.client.hpp.mako
index e5b3c9b..b6c3777 100644
--- a/tools/sdbusplus/templates/interface.client.hpp.mako
+++ b/tools/sdbusplus/templates/interface.client.hpp.mako
@@ -58,7 +58,7 @@
     template <bool, bool, bool, template <typename> typename...>
     friend class sdbusplus::async::client::client;
     template <typename>
-    friend class sdbusplus::client::${interface.cppNamespacedClass()};
+    friend struct sdbusplus::client::${interface.cppNamespacedClass()};
 
     // Delete default constructor as these should only be constructed
     // indirectly through sdbusplus::async::client_t.