x86_64: Move functions declarations to header file

Some interrupt entry points are currently defined in i8259.c They probably
belong in a header.  Right now, their only user is init_IRQ, justifying
their declaration in-file.  But when virtualization comes in, we may be
interested in using that functions in late initializations.

Signed-off-by: Glauber de Oliveira Costa <gcosta@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Andi Kleen <ak@suse.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>