stmhal/hal: do not include <stdio.h> in HAL headers

stdio.h was included in all HAL files only to provide
definition of NULL symbol

"stdio.h" includes "types.h" which contains some conflicting definitions
with "drivers/cc3000/inc/socket.h"

stmhal/hal/f4/inc/stm32f4xx_hal_def.h

@@ -47,7 +47,9 @@
 /* Includes ------------------------------------------------------------------*/
 #include "stm32f4xx.h"
 #include "Legacy/stm32_hal_legacy.h"
-#include <stdio.h>
+#ifndef NULL
+#define NULL (void*)0
+#endif
 
 /* Exported types ------------------------------------------------------------*/