Fix idle_alloc.cc build

The file did not include signal.h

Change-Id: Ie9951514a374162da149f722917fa7bdbe25e5ac
diff --git a/tools/idle_alloc.cc b/tools/idle_alloc.cc
index 09dc645..1a1dd5a 100644
--- a/tools/idle_alloc.cc
+++ b/tools/idle_alloc.cc
@@ -19,6 +19,7 @@
 #include <errno.h>
 #include <fcntl.h>
 #include <inttypes.h>
+#include <signal.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>