rust: Add missing impl_trait_in_assoc_type feature

Signed-off-by: Hector Martin <marcan@marcan.st>

Author: marcan

scripts/Makefile.build

@@ -248,7 +248,7 @@ $(obj)/%.lst: $(obj)/%.c FORCE
 # Compile Rust sources (.rs)
 # ---------------------------------------------------------------------------
 
-rust_allowed_features := allocator_api,arbitrary_self_types,lint_reasons,type_alias_impl_trait
+rust_allowed_features := allocator_api,arbitrary_self_types,impl_trait_in_assoc_type,lint_reasons,type_alias_impl_trait
 
 # `--out-dir` is required to avoid temporaries being created by `rustc` in the
 # current working directory, which may be not accessible in the out-of-tree