basic: include only what we use

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

src/basic/syslog-util.c

@@ -19,10 +19,11 @@
   along with systemd; If not, see <http://www.gnu.org/licenses/>.
 ***/
 
+#include <string.h>
 #include <syslog.h>
 
-#include "assert.h"
 #include "hexdecoct.h"
+#include "macro.h"
 #include "string-table.h"
 #include "syslog-util.h"