a11y: Fix the help text property bridging to ATSPI

It needed to be added to the vendored XML introspection data, so it was not
marked by a GDBus client as missing outright, and the comparison in the
property handler was also wrong.

gtk/a11y/atspi/Accessible.xml

@@ -16,6 +16,8 @@
 
   <property name="AccessibleId" type="s" access="read"/>
 
+  <property name="HelpText" type="s" access="read"/>
+
   <method name="GetChildAtIndex">
     <arg direction="in" name="index" type="i"/>
     <arg direction="out" type="(so)"/>

gtk/a11y/gtkatspicontext.c

@@ -729,7 +729,7 @@ handle_accessible_get_property (GDBusConnection       *connection,
     res = get_parent_context_ref (accessible);
   else if (g_strcmp0 (property_name, "ChildCount") == 0)
     res = g_variant_new_int32 (gtk_at_spi_context_get_child_count (self));
-  else if (g_strcmp0 (property_name, "HelpText"))
+  else if (g_strcmp0 (property_name, "HelpText") == 0)
     {
       if (gtk_at_context_has_accessible_property (GTK_AT_CONTEXT (self), GTK_ACCESSIBLE_PROPERTY_HELP_TEXT))
         {