fix(runtime-c-api) Provide a link target.

When compiling `wasmer-runtime-c-api` as a dependency of another
crate, `rustc` emits this warning:

```
warning: The package `wasmer_runtime_c_api` provides no linkable
target. The compiler might raise an error while compiling
`foo`. Consider adding 'dylib' or 'rlib' to key `crate-type` in
`wasmer_runtime_c_api`'s Cargo.toml. This warning might turn into a
hard error in the future.
```

To solve this issue, the `rlib` type has been added to the
`crate-type` list.

lib/runtime-c-api/Cargo.toml

@@ -14,7 +14,7 @@ wasmer-runtime-core = { path = "../runtime-core", version = "0.2.1" }
 libc = "0.2"
 
 [lib]
-crate-type = ["cdylib"]
+crate-type = ["cdylib", "rlib"]
 
 [build-dependencies]
 cbindgen = "0.8"