Update memoffset requirement from 0.2.1 to 0.5.1

Updates the requirements on [memoffset](https://github.com/Gilnaa/memoffset) to permit the latest version.
- [Release notes](https://github.com/Gilnaa/memoffset/releases)
- [Commits](https://github.com/Gilnaa/memoffset/commits/v0.5.1)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

wasmtime-runtime/Cargo.toml

@@ -20,7 +20,7 @@ region = "2.0.0"
 lazy_static = "1.2.0"
 libc = { version = "0.2.48", default-features = false }
 errno = "0.2.4"
-memoffset = "0.2.1"
+memoffset = "0.5.1"
 failure = { version = "0.1.3", default-features = false }
 failure_derive = { version = "0.1.3", default-features = false }
 indexmap = "1.0.2"