basic: include only what we use

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

src/basic/string-util.c

@@ -19,8 +19,15 @@
   along with systemd; If not, see <http://www.gnu.org/licenses/>.
 ***/
 
+#include <errno.h>
+#include <stdarg.h>
+#include <stdint.h>
+#include <stdio.h>
+#include <stdlib.h>
+
 #include "alloc-util.h"
 #include "gunicode.h"
+#include "macro.h"
 #include "string-util.h"
 #include "utf8.h"
 #include "util.h"