clang: suppress a warning

Add a missing cast.

    lib/token.c:165:46: warning: passing
          'const unsigned char *' to parameter of type 'const char *' converts
          between pointers to integer types with different sign [-Wpointer-sign]
                                                 tokenizer->next,
                                                 ^~~~~~~~~~~~~~~
    include/groonga/tokenizer.h:195:82: note:
          passing argument to parameter 'str_ptr' here
      ...const char *str_ptr,
                     ^
clang: suppress warnings

Add missing casts.

    lib/token.c:162:21: warning: assigning to
          'const unsigned char *' from 'const char *' converts between pointers to
          integer types with different sign [-Wpointer-sign]
        tokenizer->next =
                        ^
    lib/token.c:190:56: warning: passing
          'const unsigned char *' to parameter of type 'const char *' converts
          between pointers to integer types with different sign [-Wpointer-sign]
        grn_tokenizer_token_push(ctx, &(tokenizer->token), p, r - p, status);
                                                           ^
    include/groonga/tokenizer.h:183:61: note:
          passing argument to parameter 'str_ptr' here
      ...const char *str_ptr, unsigned int str_length,
                     ^