clang: suppress a warning

Use "char *" instead of "byte *" for byte string because we check only
whether any '\0' character exists or not.

    lib/str.c:3263:9: warning: initializing 'byte *'
          (aka 'unsigned char *') with an expression of type 'char *' converts
          between pointers to integer types with different sign [-Wpointer-sign]
      byte *v = GRN_BULK_HEAD(obj);
            ^   ~~~~~~~~~~~~~~~~~~