qapi: Factor out compat_policy_input_ok()

The code to check policy for handling deprecated input is triplicated.
Factor it out into compat_policy_input_ok() before I mess with it in
the next commit.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20211028102520.747396-9-armbru@redhat.com>
[Policy code moved from qmp-dispatch.c to qapi-util.c to make visitors
link without qmp-dispatch.o]

include/qapi/compat-policy.h

@@ -13,10 +13,17 @@
 #ifndef QAPI_COMPAT_POLICY_H
 #define QAPI_COMPAT_POLICY_H
 
+#include "qapi/error.h"
 #include "qapi/qapi-types-compat.h"
 
 extern CompatPolicy compat_policy;
 
+bool compat_policy_input_ok(unsigned special_features,
+                            const CompatPolicy *policy,
+                            ErrorClass error_class,
+                            const char *kind, const char *name,
+                            Error **errp);
+
 /*
  * Create a QObject input visitor for @obj for use with QMP
  *