fuzz: update libfuzzer dependency

This is intended to fix an OSS-fuzz build failure detailed here:
https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=32817

Fixes #757

Author: DavidKorczynski

fuzz/Cargo.toml

@@ -11,7 +11,7 @@ edition = "2018"
 cargo-fuzz = true
 
 [dependencies]
-libfuzzer-sys = "0.3"
+libfuzzer-sys = "0.4.0"
 
 [dependencies.regex]
 path = ".."