ext/json/parser/prereq.mk: remove type-limit warning if char is unsigned

Ragel generates a code `0 <= (*p)` where `*p` is char.
As char is unsigned by default on arm and RISC-V, it is warned by gcc:

```
compiling parser.c
parser.c: In function ‘JSON_parse_string’:
parser.c:1566:2: warning: comparison is always true due to limited range of data type [-Wtype-limits]
  if ( 0 <= (*p) && (*p) <= 31 )
  ^
parser.c:1596:2: warning: comparison is always true due to limited range of data type [-Wtype-limits]
  if ( 0 <= (*p) && (*p) <= 31 )
  ^
```

This change removes the warning by substituting the condition with
`0 <= (signed char)(*p)`.

ext/json/parser/parser.c

@@ -1563,7 +1563,7 @@ case 2:
 		case 34: goto tr2;
 		case 92: goto st3;
 	}
-	if ( 0 <= (*p) && (*p) <= 31 )
+	if ( 0 <= (signed char)(*p) && (*p) <= 31 )
 		goto st0;
 	goto st2;
 tr2:
@@ -1593,7 +1593,7 @@ st3:
 case 3:
 	if ( (*p) == 117 )
 		goto st4;
-	if ( 0 <= (*p) && (*p) <= 31 )
+	if ( 0 <= (signed char)(*p) && (*p) <= 31 )
 		goto st0;
 	goto st2;
 st4:

ext/json/parser/prereq.mk

@@ -6,6 +6,7 @@ RAGEL = ragel
 	$(RAGEL) -G2 $<
 	$(BASERUBY) -pli -e '$$_.sub!(/[ \t]+$$/, "")' \
 	-e '$$_.sub!(/^static const int (JSON_.*=.*);$$/, "enum {\\1};")' \
+	-e '$$_.sub!(/0 <= \(\*p\) && \(\*p\) <= 31/, "0 <= (signed char)(*p) && (*p) <= 31")' \
 	-e '$$_ = "/* This file is automatically generated from parser.rl by using ragel */" + $$_ if $$. == 1' $@
 
 parser.c: