fix hppfs Makefile breakage

Fallout from commit 46d7b522 ("uml: move
hppfs_kern.c to hppfs.c")

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Acked-by: Jeff Dike <jdike@addtoit.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>