remove unused includes

This patch removes includes that are not used. The removals were found with
include-what-you-use which checks if any of the symbols from a header is
in use.

src/shared/mkdir-label.c

@@ -20,16 +20,10 @@
   along with systemd; If not, see <http://www.gnu.org/licenses/>.
 ***/
 
-#include <assert.h>
-#include <string.h>
 #include <unistd.h>
-#include <errno.h>
-#include <stdlib.h>
 #include <stdio.h>
 
 #include "label.h"
-#include "util.h"
-#include "path-util.h"
 #include "mkdir.h"
 
 int mkdir_safe_label(const char *path, mode_t mode, uid_t uid, gid_t gid) {