Include kqueue.c in the Darwin build

The autoconf system correctly detected that Darwin supports kqueue,
and we need to compile this file to define some symbols declared
elsewhere behind ifdefs.

Change-Id: I754e873b8ef47d4e7a2b9d2464ddf52e7ff8b4a0
diff --git a/Android.mk b/Android.mk
index a66f968..8eaf4e5 100644
--- a/Android.mk
+++ b/Android.mk
@@ -20,6 +20,7 @@
 	select.c \
 
 libevent_darwin_src := \
+	kqueue.c \
 	poll.c \
 	select.c \