Expose the public Stream API

Stream.h contains a public API which is not currently exposed via
mbed.h (when it should be). This commit fixes this.

Author: adbridge

mbed.h

@@ -104,6 +104,7 @@
 #include "platform/ScopedRomWriteLock.h"
 #include "platform/ScopedRamExecutionLock.h"
 #include "platform/mbed_stats.h"
+#include "platform/Stream.h"
 
 // mbed Non-hardware components
 #include "platform/Callback.h"