[PATCH] powerpc: fix for kexec ppc32

- kexec.h is included from assembly code, thus C code must be properly
  protected.

- (embedded) ppc32 systems use machine_kexec_simple whose declaration
  vanished during a recent powerpc merge change.

Signed-off-by: Albert Herranz <albert_herranz@yahoo.es>
Cc: <fastboot@osdl.org>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Paul Mackerras <paulus@samba.org>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>