Add stdlib.h header where applicable (where we call malloc() / free()).

Reported by:	tinderbox

src/lib/drivers/pn531_usb.c

@@ -25,6 +25,8 @@
 Thanks to d18c7db and Okko for example code
 */
 
+#include <stdlib.h>
+
 #include "../drivers.h"
 #include <nfc/nfc-messages.h>
 

src/lib/drivers/pn533_usb.c

@@ -25,6 +25,8 @@
 Thanks to d18c7db and Okko for example code
 */
 
+#include <stdlib.h>
+
 #include "../drivers.h"
 #include <nfc/nfc-messages.h>
 

src/lib/drivers/pn53x_usb.c

@@ -27,6 +27,7 @@ Thanks to d18c7db and Okko for example code
 
 #include <sys/param.h>
 #include <stdio.h>
+#include <stdlib.h>
 #include <usb.h>
 #include <string.h>