basic: include only what we use

This is a cleaned up result of running iwyu but without forward
declarations on src/basic.

src/basic/mkdir-label.c

@@ -21,6 +21,7 @@
 ***/
 
 #include <stdio.h>
+#include <sys/types.h>
 #include <unistd.h>
 
 #include "label.h"