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: Stefan Weil <sw@weilnetz.de>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>

monitor.c

diff --git a/monitor.c b/monitor.c
index ee9744c..8c23e29 100644
--- a/monitor.c
+++ b/monitor.c
@@ -3171,7 +3171,8 @@
     { 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);