debugfs: bugfix: include <linux/io.h> in file.c

The regs32 machinery uses readl. I forgot the mandatory include
and the code was not compiling on all archs.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Alessandro Rubini <rubini@gnudd.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>