oc: Use enum mapistore_error instead of int

This specifies a little the scope of the variable to make it
more realistic with the actual values it may have. We do have
a static typed compiled language, why don't we use it?

OpenChange/MAPIStoreNotesFolder.m

@@ -30,8 +30,8 @@
 
 @implementation MAPIStoreNotesFolder
 
-- (int) getPidTagDefaultPostMessageClass: (void **) data
-                                inMemCtx: (TALLOC_CTX *) memCtx
+- (enum mapistore_error) getPidTagDefaultPostMessageClass: (void **) data
+                                                 inMemCtx: (TALLOC_CTX *) memCtx
 {
   *data = [@"IPM.StickyNote" asUnicodeInMemCtx: memCtx];