[RTOS] Fixed missing include in RtosTimer.h

Added missing include for the MBED_DEPRECATED macro.

Author: neilt6

rtos/rtos/RtosTimer.h

@@ -25,6 +25,7 @@
 #include <stdint.h>
 #include "cmsis_os.h"
 #include "Callback.h"
+#include "toolchain.h"
 
 namespace rtos {