ndk: android-9: Add missing <sys/eventfd.h> header

For some reason, the header was missing from previous NDK releases.
Note: the function symbols are properly listed in arch-$ARCH/symbols/libc.so.functions.txt

Change-Id: I72bf307ccaf55c9b8fc344f5bbfa7b44ba673d62
diff --git a/ndk/platforms/android-9/include/sys/eventfd.h b/ndk/platforms/android-9/include/sys/eventfd.h
new file mode 100644
index 0000000..19244a5
--- /dev/null
+++ b/ndk/platforms/android-9/include/sys/eventfd.h
@@ -0,0 +1,50 @@
+/*
+ * Copyright (C) 2008 The Android Open Source Project
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ *  * Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ *  * Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in
+ *    the documentation and/or other materials provided with the
+ *    distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
+ * FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
+ * COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
+ * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
+ * BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS
+ * OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED
+ * AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+ * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
+ * OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+ * SUCH DAMAGE.
+ */
+#ifndef _SYS_EVENTFD_H
+#define _SYS_EVENTFD_H
+
+#include <sys/cdefs.h>
+#include <fcntl.h>
+
+__BEGIN_DECLS
+
+#define  EFD_CLOEXEC   O_CLOEXEC
+#define  EFD_NONBLOCK  O_NONBLOCK
+
+/* type of event counter */
+typedef uint64_t  eventfd_t;
+
+extern int eventfd(unsigned int initval, int flags);
+
+/* Compatibility with GLibc */
+extern int eventfd_read(int fd, eventfd_t *counter);
+extern int eventfd_write(int fd, const eventfd_t counter);
+
+__END_DECLS
+
+#endif /* _SYS_EVENTFD_H */