[Isolates] HashMap: add missing "explicit" to the constructor with default args.

src/hashmap.h

Index: src/hashmap.h
diff --git a/src/hashmap.h b/src/hashmap.h
index 27989889c3e665a6806a61557d6f6eb13f4b011e..bb3e3ceb4496fcb85c10f7f8922dfeba76bba920 100644
--- a/src/hashmap.h
+++ b/src/hashmap.h
@@ -55,9 +55,9 @@ class HashMap {
 
   // initial_capacity is the size of the initial hash map;
   // it must be a power of 2 (and thus must not be 0).
-  HashMap(MatchFun match,
-          Allocator* allocator = &DefaultAllocator,
-          uint32_t initial_capacity = 8);
+  explicit HashMap(MatchFun match,
+                   Allocator* allocator = &DefaultAllocator,
+                   uint32_t initial_capacity = 8);
 
   ~HashMap();