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);
            ^   ~~~~~~~~~~~~~~~~~~

lib/str.c

@@ -3260,7 +3260,7 @@ grn_text_fgets(grn_ctx *ctx, grn_obj *buf, FILE *fp)
 grn_bool
 grn_bulk_is_zero(grn_ctx *ctx, grn_obj *obj)
 {
-  byte *v = GRN_BULK_HEAD(obj);
+  const char *v = GRN_BULK_HEAD(obj);
   unsigned int s = GRN_BULK_VSIZE(obj);
   for (; s; s--, v++) {
     if (*v) { return GRN_FALSE; }