basic: include only what we use

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

src/basic/hashmap.c

@@ -21,8 +21,9 @@
 ***/
 
 #include <errno.h>
-#include <pthread.h>
+#include <stdint.h>
 #include <stdlib.h>
+#include <string.h>
 
 #include "alloc-util.h"
 #include "hashmap.h"