This patch removes the `WASM_EMSCRIPTEN_GENERATE_C_API_HEADERS`
flag. Consequently, the C header files will be generated for each
build.
The `generate-c-api-headers` feature is also removed, since it becomes useless.
When requiring `wasmer-runtime-c-api`, I get the following conflicts:
```
error: failed to select a version for `serde_derive`.
... required by package `cbindgen v0.7.1`
... which is depended on by `wasmer-runtime-c-api v0.2.1`
... which is depended on by `foo v0.1.0 (/private/tmp/foo)`
versions that meet the requirements `= 1.0.58` are: 1.0.58
all possible versions conflict with previously selected packages.
previously selected package `serde_derive v1.0.89`
... which is depended on by `wasmer-runtime-core v0.2.1`
... which is depended on by `wasmer-runtime-c-api v0.2.1`
... which is depended on by `foo v0.1.0 (/private/tmp/foo)`
failed to select a version for `serde_derive` which could resolve this conflict
```
This issue resolves by updating `cbindgen` to 0.8.
