Apply const qualifier to keywords of jsonpath_scan.l

Discussion: https://postgr.es/m/CAEeOP_a-Pfy%3DU9-f%3DgQ0AsB8FrxrC8xCTVq%2BeO71-2VoWP5cag%40mail.gmail.com
Author: Mark G

src/backend/utils/adt/jsonpath_scan.l

@@ -287,7 +287,7 @@ typedef struct keyword
 	int16		len;
 	bool		lowercase;
 	int			val;
-	char	*keyword;
+	const char	*keyword;
 } keyword;
 
 /*
@@ -295,7 +295,7 @@ typedef struct keyword
  * alphabetical order
  */
 
-static keyword keywords[] = {
+static const keyword keywords[] = {
 	{ 2, false,	IS_P,		"is"},
 	{ 2, false,	TO_P,		"to"},
 	{ 3, false,	ABS_P,		"abs"},
@@ -324,7 +324,7 @@ checkSpecialVal()
 {
 	int				res = IDENT_P;
 	int				diff;
-	keyword		*StopLow = keywords,
+	const keyword  *StopLow = keywords,
 				   *StopHigh = keywords + lengthof(keywords),
 				   *StopMiddle;