Fix code generation when using LITE_RUNTIME and enums

input file:
```proto
option optimize_for = LITE_RUNTIME;

enum MemberType {
    NODE = 0;
    WAY = 1;
}
```
codegen was generating `fn enum_descriptor_static` with LITE_RUNTIME, but with
this option, `fn file_descriptor_proto` was not generated.  It's fixed by not
generating `fn enum_descriptor_static` for `enum` when LITE_RUNTIME is enable.