basic: include only what we use

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

src/basic/stat-util.c

@@ -19,7 +19,11 @@
   along with systemd; If not, see <http://www.gnu.org/licenses/>.
 ***/
 
+#include <dirent.h>
+#include <errno.h>
 #include <fcntl.h>
+#include <sys/stat.h>
+#include <sys/types.h>
 #include <linux/magic.h>
 #include <sys/statvfs.h>
 #include <unistd.h>