x86: k8.h: Add struct bootnode

k8.h uses struct bootnode but does not #include a header file
for it, so provide a simple declaration for it.

  arch/x86/include/asm/k8.h:13: warning: 'struct bootnode'
  declared inside parameter list arch/x86/include/asm/k8.h:13:
  warning: its scope is only this definition or declaration, which is probably not what you want

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Acked-by: David Rientjes <rientjes@google.com>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>
LKML-Reference: <20091028160955.d27ccb16.randy.dunlap@oracle.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>