x86: process.c declare c1e_remove_cpu before they get used

Impact: cleanup, avoid sparse warning

Included asm/idle.h for c1e_remove_cpu() declaration. Fixes this
sparse warning:

  CHECK   arch/x86/kernel/process.c
  arch/x86/kernel/process.c:284:6: warning: symbol 'c1e_remove_cpu' was not declared. Should it be static?

Signed-off-by: Jaswinder Singh <jaswinder@infradead.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>