x86/build: Add arch/x86/purgatory/ make generated files to gitignore

The following generated files are missing from gitignore
and show up in git status after x86_64 build. Add them
to gitignore.

    arch/x86/purgatory/kexec-purgatory.c
    arch/x86/purgatory/purgatory.ro

Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>
Link: http://lkml.kernel.org/r/1412016116-7213-1-git-send-email-shuahkh@osg.samsung.com


Signed-off-by: Ingo Molnar <mingo@kernel.org>