Tag ALL my_error_reporters with ATTRIBUTE_FORMAT

The function pointer typedef `my_error_reporter` is already tagged.
This commit inherits this attribute to all `my_getopt_error_reporter`s
and `my_charset_error_reporter`s for consistency.
(It future-proofs for deliberate direct uses of those functions.)

mysys/charset.c

@@ -477,7 +477,7 @@ static int add_collation(struct charset_info_st *cs)
   Report character set initialization errors and warnings.
   Be silent by default: no warnings on the client side.
 */
-static void
+ATTRIBUTE_FORMAT(printf, 2, 3) static void
 default_reporter(enum loglevel level  __attribute__ ((unused)),
                  const char *format  __attribute__ ((unused)),
                  ...)

mysys/my_getopt.c

@@ -86,6 +86,7 @@ my_bool my_handle_options_init_variables = 1;
 
 my_getopt_value my_getopt_get_addr= 0;
 
+ATTRIBUTE_FORMAT(printf, 2, 3)
 static void default_reporter(enum loglevel level, const char *format, ...)
 {
   va_list args;

sql/mysqld.cc

@@ -1435,6 +1435,7 @@ C_MODE_START
   @param level          log message level
   @param format         log message format string
 */
+ATTRIBUTE_FORMAT(printf, 2, 3)
 static void buffered_option_error_reporter(enum loglevel level,
                                            const char *format, ...)
 {
@@ -1465,6 +1466,7 @@ static void buffered_option_error_reporter(enum loglevel level,
   to print general character set subsystem initialization errors,
   such as Index.xml syntax problems, bad XML tag hierarchy, etc.
 */
+ATTRIBUTE_FORMAT(printf, 2, 3)
 static void charset_error_reporter(enum loglevel level,
                                    const char *format, ...)
 {
@@ -8720,7 +8722,7 @@ mysql_getopt_value(const char *name, uint length,
   return option->value;
 }
 
-
+ATTRIBUTE_FORMAT(printf, 2, 3)
 static void option_error_reporter(enum loglevel level, const char *format, ...)
 {
   va_list args;