do_open_proc: annotate with printf attr

This will help us catch bugs where the arguments are not in line
with the format string.
Signed-off-by: Kir Kolyshkin <kir@openvz.org>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>

criu/include/util.h

@@ -73,7 +73,8 @@ extern int set_proc_fd(int fd);
 #define PROC_GEN	-1
 #define PROC_NONE	-2
 
-extern int do_open_proc(pid_t pid, int flags, const char *fmt, ...);
+extern int do_open_proc(pid_t pid, int flags, const char *fmt, ...)
+	__attribute__ ((__format__ (__printf__, 3, 4)));
 
 #define __open_proc(pid, ier, flags, fmt, ...)				\
 	({								\