x86: prevent stale state of c1e_mask across CPU offline/online, fix

Fix build error introduced by commit 4faac97d ("x86: prevent stale
state of c1e_mask across CPU offline/online").

process_32.c needs to include idle.h to get the prototype for
c1e_remove_cpu()

Signed-off-by: Marc Dionne <marc.c.dionne@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>