Fix filesystem-disk.c++ build under C++11.

c++/src/kj/filesystem.h

@@ -287,6 +287,12 @@ public:
     // - Access control info: Differs wildly across platforms, and KJ prefers capabilities anyway.
     // - Other timestamps: Differs across platforms.
     // - Device number: If you care, you're probably doing platform-specific stuff anyway.
+
+    Metadata() = default;
+    Metadata(Type type, uint64_t size, uint64_t spaceUsed, Date lastModified, uint linkCount)
+        : type(type), size(size), spaceUsed(spaceUsed), lastModified(lastModified),
+          linkCount(linkCount) {}
+    // TODO(cleanup): This constructor is redundant in C++14, but needed in C++11.
   };
 
   virtual Metadata stat() = 0;