libc: add <sys/eventfd.h> and corresponding implementations.

Change-Id: Ide040884c456190226e580513099fdb8377e015b
diff --git a/libc/Android.mk b/libc/Android.mk
index 9d7dd3d..9e6bdfb 100644
--- a/libc/Android.mk
+++ b/libc/Android.mk
@@ -11,6 +11,7 @@
 	unistd/brk.c \
 	unistd/creat.c \
 	unistd/daemon.c \
+	unistd/eventfd.c \
 	unistd/exec.c \
 	unistd/fcntl.c \
 	unistd/fnmatch.c \