Revert "tools: ynl: Remove duplicated include in handshake-user.c"

This reverts commit e7c5433c.

Commit e7c5433c ("tools: ynl: Remove duplicated include
in handshake-user.c") was applied too hastily. It changes
an auto-generated file, and there's already a proper fix
on the list.

Link: https://lore.kernel.org/all/ZIMPLYi%2FxRih+DlC@nanopsycho/Signed-off-by: Jakub Kicinski <kuba@kernel.org>

tools/net/ynl/generated/handshake-user.c

@@ -8,6 +8,7 @@
 #include "ynl.h"
 #include <linux/handshake.h>
 
+#include <stdlib.h>
 #include <stdio.h>
 #include <string.h>
 #include <libmnl/libmnl.h>