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>

fs/debugfs/file.c

@@ -19,6 +19,7 @@
 #include <linux/pagemap.h>
 #include <linux/namei.h>
 #include <linux/debugfs.h>
+#include <linux/io.h>
 
 static ssize_t default_read_file(struct file *file, char __user *buf,
 				 size_t count, loff_t *ppos)