rust: Allow for usage in no_std environment (#6989)

rust/flatbuffers/src/lib.rs

@@ -28,6 +28,11 @@
 //! At this time, to generate Rust code, you will need the latest `master` version of `flatc`, available from here: <https://github.com/google/flatbuffers>
 //! (On OSX, you can install FlatBuffers from `HEAD` with the Homebrew package manager.)
 
+#![cfg_attr(feature = "no_std", no_std)]
+
+#[cfg(feature = "no_std")]
+extern crate alloc;
+
 mod array;
 mod builder;
 mod endian_scalar;