Fixed warning about discarding modifiers.

```
../src/ruby_parser.c:677:48: warning: passing 'rb_encoding *' (aka 'const struct OnigEncodingTypeST *') to parameter of type 'void *' discards qualifiers [-Wincompatible-pointer-types-discards-qualifiers]
  677 |     ast = rb_parser_compile(p, gets, ptr, len, enc, input, line);
      |                                                ^~~
../src/internal/parse.h:58:128: note: passing argument to parameter 'fname_enc' here
   58 | rb_ast_t *rb_parser_compile(rb_parser_t *p, rb_parser_lex_gets_func *gets, const char *fname_ptr, long fname_len, rb_encoding *fname_enc, rb_parser_input_data input, int line);
      |                                                                                                                                ^
```