Add format checking to RawSerial::printf

drivers/RawSerial.h

@@ -20,6 +20,7 @@
 
 #if defined (DEVICE_SERIAL) || defined(DOXYGEN_ONLY)
 
+#include "mbed_toolchain.h"
 #include "drivers/SerialBase.h"
 #include "hal/serial_api.h"
 #include "platform/NonCopyable.h"
@@ -86,7 +87,7 @@ public:
      */
     int puts(const char *str);
 
-    int printf(const char *format, ...);
+    int printf(const char *format, ...) MBED_PRINTF_METHOD(1, 2);
 
 #if !(DOXYGEN_ONLY)
 protected: