hw/ppc/spapr: Include missing 'sysemu/tcg.h' header

hw/ppc/spapr: Include missing 'sysemu/tcg.h' header

"sysemu/tcg.h" declares tcg_enabled(), and is implicitly included.
Include it explicitly to avoid the following error when refactoring
headers:

  hw/ppc/spapr.c:2612:9: error: call to undeclared function 'tcg_enabled'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
    if (tcg_enabled()) {
        ^

Reviewed-by: Harsh Prateek Bora <harshpb@linux.ibm.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>