busybox: add upstream 1.16 fix

Fixes https://bugs.busybox.net/show_bug.cgi?id=1177, available at
http://busybox.net/downloads/fixes-1.16.0/busybox-1.16.0-fsync.patch

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Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>