m32r: build fix for removing fs.h from mm.h

This patch fixes the m32r build after the commit "Remove fs.h from mm.h"
was merged (commit 4e950f6f).

Signed-off-by: Hirokazu Takata <takata@linux-m32r.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>