x86/irqflags: Mark native_restore_fl extern inline

This should have been marked extern inline in order to pick up the out
of line definition in arch/x86/kernel/irqflags.S.

Fixes: 208cbb32

 ("x86/irqflags: Provide a declaration for native_save_fl")
Reported-by: Ben Hutchings <ben.hutchings@codethink.co.uk>
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Juergen Gross <jgross@suse.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: stable@vger.kernel.org
Link: https://lkml.kernel.org/r/20180827214011.55428-1-ndesaulniers@google.com