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

arch/x86/kernel/process_32.c

@@ -55,6 +55,7 @@
 #include <asm/tlbflush.h>
 #include <asm/cpu.h>
 #include <asm/kdebug.h>
+#include <asm/idle.h>
 
 asmlinkage void ret_from_fork(void) __asm__("ret_from_fork");