src: consistently declare BindingData class

Its defined as a class, so forward declare as a class, fixing type
mismatch warning.

PR-URL: nodejs#32677
Reviewed-By: Anna Henningsen <[email protected]>
Reviewed-By: Richard Lau <[email protected]>

src/node_stat_watcher.h

@@ -31,7 +31,7 @@
 
 namespace node {
 namespace fs {
-struct BindingData;
+class BindingData;
 }
 
 class Environment;