kexec: define kexec_in_progress in !CONFIG_KEXEC case

If some piece of code wants to check kexec_in_progress it has to be put
in #ifdef CONFIG_KEXEC block to not break the build in !CONFIG_KEXEC
case. Overcome this limitation by defining kexec_in_progress to false.

Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>