Include <stdlib.h>, required for getenv(3).

Fix build if -Werror is set, avoid coredumping at runtime otherwise.

Reported by:	Ganael Laplanche

libnfc/buses/usbbus.c

@@ -33,6 +33,8 @@
 #  include "config.h"
 #endif // HAVE_CONFIG_H
 
+#include <stdlib.h>
+
 #include "usbbus.h"
 #include "log.h"
 #define LOG_CATEGORY "libnfc.buses.usbbus"