ipa: ipa_interface: Add comment to defend the forward-declaration

Any struct that is defined in core.mojom with the skipHeader tag needs
to be present in ipa_interface.h, either forward-declared or #included.
Add a comment so that, in the future, people don't try to send patches
removing the seemingly unused forward-declaration.

Signed-off-by: Paul Elder <paul.elder@ideasonboard.com>
Reviewed-by: Niklas Söderlund <niklas.soderlund@ragnatech.se>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>

include/libcamera/ipa/ipa_interface.h

@@ -20,6 +20,11 @@
 
 namespace libcamera {
 
+/*
+ * Structs that are defined in core.mojom and have the skipHeader tag must be
+ * forward-declared or #included here.
+ */
+
 struct CameraSensorInfo;
 
 class IPAInterface