hw/ppc/spapr: Reduce "vof.h" inclusion

hw/ppc/spapr: Reduce "vof.h" inclusion

Currently objects including "hw/ppc/spapr.h" are forced to be
target specific due to the inclusion of "vof.h" in "spapr.h".

"spapr.h" only uses a Vof pointer, so doesn't require the structure
declaration. The only place where Vof structure is accessed is in
spapr.c, so include "vof.h" there, and forward declare the structure
in "spapr.h".

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Message-Id: <20221213123550.39302-4-philmd@linaro.org>
Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>