clang: suppress a warning

Add missing cast.

    lib/token.c:61:19: warning: assigning to 'byte *'
          (aka 'unsigned char *') from 'char *' converts between pointers to integer
          types with different sign [-Wpointer-sign]
      tokenizer->curr = GRN_TEXT_VALUE(str);
                      ^ ~~~~~~~~~~~~~~~~~~~
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,
                     ^