libxl: fix missing include of <signal.h>

libxl_dm.c:713: warning: implicit declaration of function 'kill'

Signed-off-by: Christoph Egger <Christoph.Egger@amd.com>
Committed-by: Ian Jackson <ian.jackson@eu.citrix.com>

tools/libxl/libxl_dm.c

@@ -20,6 +20,7 @@
 #include <stdio.h>
 #include <string.h>
 #include <stdlib.h>
+#include <signal.h>
 #include <unistd.h>
 #include <fcntl.h>
 #include "libxl_utils.h"