Fix build with clang-15+

Fixes
json_util.c:63:35: error: a function declaration without a prototype is deprecated in all versions of C [-We
rror,-Wstrict-prototypes]
const char *json_util_get_last_err()
                                  ^
                                   void

Signed-off-by: Khem Raj <raj.khem@gmail.com>

tests/test4.c

@@ -31,7 +31,7 @@ void print_hex(const char *s)
 }
 
 static void test_lot_of_adds(void);
-static void test_lot_of_adds()
+static void test_lot_of_adds(void)
 {
 	int ii;
 	char key[50];