cpu-common.h: Remove unnecessary guard on including targphys.h

There's no need to make the include of targphys.h conditional
on whether TARGET_PHYS_ADDR_BITS is defined, because targphys.h
itself checks that and does nothing if it isn't.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Andreas Färber <afaerber@suse.de>
Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
