Add a missing #include.

Change-Id: Iebb2d6d26beaaac09dece87bbe2165f52f0b63eb
diff --git a/src/dalvik_system_Zygote.cc b/src/dalvik_system_Zygote.cc
index d74d3eb..49556bf 100644
--- a/src/dalvik_system_Zygote.cc
+++ b/src/dalvik_system_Zygote.cc
@@ -16,6 +16,7 @@
 
 #include <grp.h>
 #include <paths.h>
+#include <signal.h>
 #include <stdlib.h>
 #include <sys/prctl.h>
 #include <sys/types.h>