uml: fix build for !CONFIG_PRINTK

Handle the case of CONFIG_PRINTK being disabled.  This requires a do-nothing
stub to be present in arch/um/include/user.h so that we don't get references
to printk from libc code.

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>