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I didn't find any functions used for which the documentation specifies to
use (at least) one of the dropped headers.

The only Linux specific header <linux/fs.h> is needed for BLKGETSIZE,
document that.
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Chris Ball <chris@printf.net>