F4_HAL/def: Define NULL instead of including stdio.h.

The definition of NULL is the only reason stdio.h is needed, and including
this standard header clashes with a CC3000 driver's definitions because
the latter wants to redefine built-in macros.  The patch here is the
simplest way to deal with this but the proper fix is really to modify the
CC3000 driver.

STM32F4xx_HAL_Driver/Inc/stm32f4xx_hal_def.h

@@ -47,7 +47,11 @@
 /* Includes ------------------------------------------------------------------*/
 #include "stm32f4xx.h"
 #include "Legacy/stm32_hal_legacy.h"
-#include <stdio.h>
+
+// instead of including stdio.h
+#ifndef NULL
+#define NULL ((void*)0)
+#endif
 
 /* Exported types ------------------------------------------------------------*/