Make c-exp.y work with Bison 3.8+

When using Bison 3.8, we get this error:
 ../../gdb/c-exp.y:3455:1: error: ‘void c_print_token(FILE*, int, YYSTYPE)’ defined but not used [-Werror=unused-function]

That's because bison 3.8 removed YYPRINT support:
https://savannah.gnu.org/forum/forum.php?forum_id=10047

Accordingly, this patch only defines that function for Bison < 3.8.

---
 gdb/c-exp.y | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/gdb/c-exp.y b/gdb/c-exp.y
index 93b10f05b7d..9b4b88accfe 100644
--- a/gdb/c-exp.y
+++ b/gdb/c-exp.y
@@ -167,7 +167,7 @@ static struct stoken operator_stoken (const char *);
 static struct stoken typename_stoken (const char *);
 static void check_parameter_typelist (std::vector<struct type *> *);
 
-#ifdef YYBISON
+#if defined(YYBISON) && YYBISON < 30800
 static void c_print_token (FILE *file, int type, YYSTYPE value);
 #define YYPRINT(FILE, TYPE, VALUE) c_print_token (FILE, TYPE, VALUE)
 #endif
@@ -3446,7 +3446,8 @@ c_parse (struct parser_state *par_state)
   return result;
 }
 
-#ifdef YYBISON
+#if defined(YYBISON) && YYBISON < 30800
+
 
 /* This is called via the YYPRINT macro when parser debugging is
    enabled.  It prints a token's value.  */