basic: include only what we use

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

src/basic/hostname-util.c

@@ -19,14 +19,19 @@
   along with systemd; If not, see <http://www.gnu.org/licenses/>.
 ***/
 
-#include <ctype.h>
+#include <bits/local_lim.h>
+#include <errno.h>
+#include <limits.h>
+#include <stdio.h>
+#include <string.h>
 #include <sys/utsname.h>
+#include <unistd.h>
 
 #include "fd-util.h"
 #include "fileio.h"
 #include "hostname-util.h"
+#include "macro.h"
 #include "string-util.h"
-#include "util.h"
 
 bool hostname_is_set(void) {
         struct utsname u;