Force fuzzing with 1 codegen unit

This is a workaround for an LLVM 8 codegen issue. See
rust-lang/rust#53945 for more information.

Author: garyttierney

.travis.yml

@@ -23,6 +23,7 @@ script:
   - |
     if [ "$TRAVIS_PULL_REQUEST" = "false" ]; then
       pushd secsp_fuzzer/
+      export RUSTFLAGS='-C codegen-units=1'
       cargo install afl
       cargo afl build
       timeout 10m cargo afl fuzz -i ../examples -o out target/debug/secsp-fuzzer > /dev/null || true