monitor: Add missing attributes to local function

Function expr_error gets a format string and variable arguments like printf.
It also never returns. Add the necessary attributes.
Signed-off-by: NStefan Weil <sw@weilnetz.de>
Signed-off-by: NLuiz Capitulino <lcapitulino@redhat.com>

monitor.c

@@ -3171,7 +3171,8 @@ static const MonitorDef monitor_defs[] = {
     { NULL },
 };
 
-static void expr_error(Monitor *mon, const char *fmt, ...)
+static void GCC_FMT_ATTR(2, 3) QEMU_NORETURN
+expr_error(Monitor *mon, const char *fmt, ...)
 {
     va_list ap;
     va_start(ap, fmt);