[6.0] 33fcdd966 Add missing include

[Dridi Boukelmoune]

commit 33fcdd966aa11ea4f933b9415141e19048459790
Author: Federico G. Schwindt <fgsch at lodoss.net>
Date:   Wed Nov 14 06:55:09 2018 +0900

    Add missing include

diff --git a/bin/varnishadm/varnishadm.c b/bin/varnishadm/varnishadm.c
index 69f319509..750f5ddf7 100644
--- a/bin/varnishadm/varnishadm.c
+++ b/bin/varnishadm/varnishadm.c
@@ -53,6 +53,7 @@
 #include <math.h>
 #include <fcntl.h>
 #include <poll.h>
+#include <signal.h>
 #include <stdint.h>
 #include <stdlib.h>
 #include <string.h>