hw: Define empty HW_COMPAT_2_[23] macros

Now we can make everything consistent and define the macros even if they
are still empty.

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

include/hw/compat.h

 #ifndef HW_COMPAT_H
 #define HW_COMPAT_H
 
+#define HW_COMPAT_2_3 \
+        /* empty */
+
+#define HW_COMPAT_2_2 \
+        /* empty */
+
 #define HW_COMPAT_2_1 \
         {\
             .driver   = "intel-hda",\