basic: include only what we use

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

src/basic/strbuf.c

@@ -19,12 +19,12 @@
   along with systemd; If not, see <http://www.gnu.org/licenses/>.
 ***/
 
+#include <errno.h>
 #include <stdlib.h>
 #include <string.h>
 
 #include "alloc-util.h"
 #include "strbuf.h"
-#include "util.h"
 
 /*
  * Strbuf stores given strings in a single continuous allocated memory