src: consistently declare BindingData class

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

PR-URL: https://github.com/nodejs/node/pull/32677
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>

src/node_stat_watcher.h

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