libsysprof-profile: add API to list required policy

The idea here is that we stop having instruments do their own policy
checking and instead do the policy checking as a set from the recording
as part of prepare/etc.

src/libsysprof-profile/sysprof-instrument-private.h

@@ -34,6 +34,10 @@ struct _SysprofInstrument
 struct _SysprofInstrumentClass
 {
   GObjectClass parent_class;
+
+  char **(*list_required_policy) (SysprofInstrument *self);
 };
 
+char **_sysprof_instrument_list_required_policy (SysprofInstrument *self);
+
 G_END_DECLS