basic: include only what we use

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

src/basic/device-nodes.h

@@ -21,6 +21,7 @@
   along with systemd; If not, see <http://www.gnu.org/licenses/>.
 ***/
 
+#include <stddef.h>
 #include <sys/types.h>
 
 int encode_devnode_name(const char *str, char *str_enc, size_t len);