[PATCH] uml: ifdef a mode-specific function

uml_idle_timer is tt-mode only, so ifdef it as such to make it easier to spot
when tt mode is killed.

Signed-off-by: Jeff Dike <jdike@addtoit.com>
Cc: Paolo 'Blaisorblade' Giarrusso <blaisorblade@yahoo.it>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>