Fixed indirect-dependency of mbed-client-classic through Socket.h

Before:
m2mconnectionhandlerpimpl.cpp -> Socket.h -> mbed.h

Now:
m2mconnectionhandlerpimpl.cpp -> mbed.h

Author: geky

features/FEATURE_CLIENT/mbed-client-classic/source/m2mconnectionhandlerpimpl.cpp

@@ -27,6 +27,7 @@
 #include "eventOS_scheduler.h"
 
 #include "mbed-trace/mbed_trace.h"
+#include "mbed.h"
 
 #define TRACE_GROUP "mClt"