readline: Add missing GCC_FMT_ATTR

This fixes a compiler warning with -Werror=missing-format-attribute
and allows improved compiler checks for variable argument lists.
Signed-off-by: NStefan Weil <sw@weilnetz.de>
Reviewed-by: NStefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: NMichael Tokarev <mjt@tls.msk.ru>

include/qemu/readline.h

@@ -5,7 +5,8 @@
 #define READLINE_MAX_CMDS 64
 #define READLINE_MAX_COMPLETIONS 256
 
-typedef void ReadLinePrintfFunc(void *opaque, const char *fmt, ...);
+typedef void GCC_FMT_ATTR(2, 3) ReadLinePrintfFunc(void *opaque,
+                                                   const char *fmt, ...);
 typedef void ReadLineFlushFunc(void *opaque);
 typedef void ReadLineFunc(void *opaque, const char *str,
                           void *readline_opaque);

monitor.c

@@ -4888,7 +4888,8 @@ static void sortcmdlist(void)
 /* These functions just adapt the readline interface in a typesafe way.  We
  * could cast function pointers but that discards compiler checks.
  */
-static void monitor_readline_printf(void *opaque, const char *fmt, ...)
+static void GCC_FMT_ATTR(2, 3) monitor_readline_printf(void *opaque,
+                                                       const char *fmt, ...)
 {
     va_list ap;
     va_start(ap, fmt);

qemu-io.c

@@ -219,7 +219,8 @@ static char *get_prompt(void)
     return prompt;
 }
 
-static void readline_printf_func(void *opaque, const char *fmt, ...)
+static void GCC_FMT_ATTR(2, 3) readline_printf_func(void *opaque,
+                                                    const char *fmt, ...)
 {
     va_list ap;
     va_start(ap, fmt);