uml: remove include of asm/user.h

I allowed an include of asm/user.h to sneak back in.  This patch replaces
it with sys/user.h.

Signed-off-by: Jeff Dike <jdike@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>