Merge pull request #9986 from kaspar030/saul_stddef_include

sys: saul: add missing stddef.h include (for NULL)

sys/saul_reg/saul_reg.c

@@ -18,6 +18,7 @@
  * @}
  */
 
+#include <stddef.h>
 #include <stdio.h>
 #include <stdint.h>
 #include <string.h>

sys/shell/commands/sc_saul_reg.c

@@ -19,6 +19,7 @@
  * @}
  */
 
+#include <stddef.h>
 #include <stdio.h>
 #include <string.h>
 #include <stdlib.h>