sys/pipe: always include stdlib.h instead of malloc.h

The malloc.h file is deprecated on most systems and should not be
used, instead include stdlib.h which handles needed function in a
portable manner.

sys/pipe/pipe_dynamic.c

@@ -25,11 +25,7 @@
  * @}
  */
 
-#if defined(MCU_ATMEGA2560) || defined(MCU_ATMEGA1281) || defined(MCU_ATMEGA328P)
 #include <stdlib.h>
-#else
-#include <malloc.h>
-#endif
 
 #include "pipe.h"