initctl: Add missing signal.h include for older systems, e.g. Travis CI

Signed-off-by: Joachim Nilsson <troglobit@gmail.com>

initctl.c

@@ -26,6 +26,7 @@
 #include <getopt.h>
 #include <glob.h>
 #include <paths.h>
+#include <signal.h>
 #include <stdio.h>
 #include <arpa/inet.h>
 #include <sys/socket.h>