clang: suppress warnings

Add missing casts.

    lib/token.c:74:30: warning: passing 'byte *'
          (aka 'unsigned char *') to parameter of type 'const char *' converts
          between pointers to integer types with different sign [-Wpointer-sign]
                                 tokenizer->curr, 0,
                                 ^~~~~~~~~~~~~~~
    include/groonga/tokenizer.h:183:61: note:
          passing argument to parameter 'str_ptr' here
      ...const char *str_ptr, unsigned int str_length,
                     ^
    lib/token.c:84:30: warning: passing 'byte *'
          (aka 'unsigned char *') to parameter of type 'const char *' converts
          between pointers to integer types with different sign [-Wpointer-sign]
                                 tokenizer->curr, tokenizer->unit,
                                 ^~~~~~~~~~~~~~~
    include/groonga/tokenizer.h:183:61: note:
          passing argument to parameter 'str_ptr' here
      ...const char *str_ptr, unsigned int str_length,
                     ^

lib/token.c

@@ -71,7 +71,7 @@ uvector_next(grn_ctx *ctx, int nargs, grn_obj **args, grn_user_data *user_data)
   byte *p = tokenizer->curr + tokenizer->unit;
   if (tokenizer->tail < p) {
     grn_tokenizer_token_push(ctx, &(tokenizer->token),
-                             tokenizer->curr, 0,
+                             (const char *)tokenizer->curr, 0,
                              GRN_TOKENIZER_TOKEN_LAST);
   } else {
     grn_tokenizer_status status;
@@ -81,7 +81,7 @@ uvector_next(grn_ctx *ctx, int nargs, grn_obj **args, grn_user_data *user_data)
       status = GRN_TOKENIZER_TOKEN_CONTINUE;
     }
     grn_tokenizer_token_push(ctx, &(tokenizer->token),
-                             tokenizer->curr, tokenizer->unit,
+                             (const char *)tokenizer->curr, tokenizer->unit,
                              status);
     tokenizer->curr = p;
   }