Fix warnings in forward declarations

When compiled with clang (or possibly gcc with the right warnings
enabled) sdbusplus throws warnings on bus and slot, because the forward
declaration declares them as class, when they are in fact struct when
they're finally defined.

This commit resolves the declarations to match their actual definitions,
and resolves the warning.

Tested By:
Compiled bmcweb with clang, -Wall and -Werror, and compile succeeded

Change-Id: Id594669d21a296c65ee2b96f3502e692e451da4c
Signed-off-by: Ed Tanous <ed.tanous@intel.com>
diff --git a/sdbusplus/bus.hpp.in b/sdbusplus/bus.hpp.in
index 76b1566..90b7803 100644
--- a/sdbusplus/bus.hpp.in
+++ b/sdbusplus/bus.hpp.in
@@ -51,7 +51,7 @@
 {
 
 using busp_t = sd_bus*;
-class bus;
+struct bus;
 
 /** @brief Get an instance of the 'default' bus. */
 bus new_default();
diff --git a/sdbusplus/slot.hpp b/sdbusplus/slot.hpp
index 30a29c3..557d0e3 100644
--- a/sdbusplus/slot.hpp
+++ b/sdbusplus/slot.hpp
@@ -11,7 +11,7 @@
 {
 
 using slotp_t = sd_bus_slot*;
-class slot;
+struct slot;
 
 namespace details
 {