Add some missing header inclusions

index_set.hpp

@@ -19,6 +19,7 @@
 #ifndef REALM_INDEX_SET_HPP
 #define REALM_INDEX_SET_HPP
 
+#include <cstdlib>
 #include <vector>
 
 namespace realm {

schema.hpp

@@ -19,6 +19,7 @@
 #ifndef REALM_SCHEMA_HPP
 #define REALM_SCHEMA_HPP
 
+#include <string>
 #include <vector>
 
 namespace realm {

shared_realm.hpp

@@ -19,12 +19,13 @@
 #ifndef REALM_REALM_HPP
 #define REALM_REALM_HPP
 
+#include "object_store.hpp"
+
 #include <memory>
+#include <mutex>
 #include <thread>
 #include <vector>
 
-#include "object_store.hpp"
-
 namespace realm {
     class ClientHistory;
     class Realm;