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-table.h

@@ -22,6 +22,7 @@
   along with systemd; If not, see <http://www.gnu.org/licenses/>.
 ***/
 
+#include <errno.h>
 #include <stddef.h>
 #include <stdio.h>
 #include <string.h>