Apply --cfg=fuzzing when fuzzing, as required by new secp256k1

Note that rust-fuzz wrappers (including honggfuzz) already apply
this for us.

Author: TheBlueMatt

.github/workflows/build.yml

@@ -202,7 +202,7 @@ jobs:
           sudo apt-get update
           sudo apt-get -y install build-essential binutils-dev libunwind-dev
       - name: Sanity check fuzz targets on Rust ${{ env.TOOLCHAIN }}
-        run: cd fuzz && cargo test --verbose --color always
+        run: cd fuzz && RUSTFLAGS="--cfg=fuzzing" cargo test --verbose --color always
       - name: Run fuzzers
         run: cd fuzz && ./ci-fuzz.sh