eventpoll.h: fix epoll event masks

[resend due to me forgetting to cc: linux-api the first time around I
posted these back on Feb 23]

From: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

When userspace tries to use these defines, it complains that it needs to
be an unsigned 1 that is shifted, so libc implementations have to create
their own version.  Fix this by defining it properly so that libcs can
just use the kernel uapi header.

Reported-by: Elliott Hughes <enh@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>