SdBusImpl: Expose the global sdbus_impl

Ultimately we want to use this throughout the sdbusplus headers for
dependency injection in the default case. We are already defining
it manually in <sdbusplus/bus.hpp> and <sdbusplus/message.hpp>. It would
make more sense to export it from <sdbusplus/sdbus.hpp> so do that.

Tested:
    Test suite still builds and passes.

Change-Id: I126864c6ff0c35e37f2e002247f94fea0688d918
Signed-off-by: William A. Kennington III <wak@google.com>
diff --git a/sdbusplus/bus.hpp.in b/sdbusplus/bus.hpp.in
index 638a515..ddaeb8f 100644
--- a/sdbusplus/bus.hpp.in
+++ b/sdbusplus/bus.hpp.in
@@ -11,8 +11,6 @@
 #include <sdbusplus/message.hpp>
 #include <sdbusplus/sdbus.hpp>
 
-extern sdbusplus::SdBusImpl sdbus_impl;
-
 namespace sdbusplus
 {
 
diff --git a/sdbusplus/message.hpp b/sdbusplus/message.hpp
index 2c66d06..6a3661f 100644
--- a/sdbusplus/message.hpp
+++ b/sdbusplus/message.hpp
@@ -8,8 +8,6 @@
 #include <sdbusplus/message/native_types.hpp>
 #include <sdbusplus/sdbus.hpp>
 
-extern sdbusplus::SdBusImpl sdbus_impl;
-
 namespace sdbusplus
 {
 
diff --git a/sdbusplus/sdbus.cpp b/sdbusplus/sdbus.cpp
index 45a68ba..cc253e7 100644
--- a/sdbusplus/sdbus.cpp
+++ b/sdbusplus/sdbus.cpp
@@ -1,3 +1,6 @@
 #include <sdbusplus/sdbus.hpp>
 
-sdbusplus::SdBusImpl sdbus_impl;
+namespace sdbusplus
+{
+SdBusImpl sdbus_impl;
+}
diff --git a/sdbusplus/sdbus.hpp b/sdbusplus/sdbus.hpp
index 2b77271..453194b 100644
--- a/sdbusplus/sdbus.hpp
+++ b/sdbusplus/sdbus.hpp
@@ -387,4 +387,6 @@
     }
 };
 
+extern SdBusImpl sdbus_impl;
+
 } // namespace sdbusplus