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.c

@@ -19,6 +19,7 @@
   along with systemd; If not, see <http://www.gnu.org/licenses/>.
 ***/
 
+#include "string-util.h"
 #include "string-table.h"
 
 ssize_t string_table_lookup(const char * const *table, size_t len, const char *key) {