Set cfg=fuzzing when building fuzz crate in CI

We will likely drop the fuzztarget feature soon, and should thus be
setting cfg=fuzzing explicitly anyway.

Author: TheBlueMatt

ci/check-compiles.sh

@@ -5,5 +5,5 @@ echo Testing $(git log -1 --oneline)
 cargo check
 cargo doc
 cargo doc --document-private-items
-cd fuzz && cargo check --features=stdin_fuzz
+cd fuzz && RUSTFLAGS="--cfg=fuzzing" cargo check --features=stdin_fuzz
 cd ../lightning && cargo check --no-default-features --features=no-std