meta-agl-flutter: mask out libstd-rs bbappend

meta-agl-flutter: mask out libstd-rs bbappend

The libstd-rs bbappend in meta-flutter breaks building with newer
versions of Rust, so BBMASK it out for now.  Since the change it
makes is to enable debugging Dart/Rust FFI wrappers, and AGL does
not currently use such functionality, this should not change any
behavior.

Bug-AGL: SPEC-5070

Change-Id: I6984303bfc478a7574992f3d927bdaaeb41afa7c
Signed-off-by: Scott Murray <scott.murray@konsulko.com>
(cherry picked from commit 3058f411221cebcd4f97941d8e9a4c7d65a193b1)