ppc: Fix duplicated typedefs to be able to compile with Clang in gnu99 mode

ppc: Fix duplicated typedefs to be able to compile with Clang in gnu99 mode

When compiling the ppc code with clang and -std=gnu99, there are a
couple of warnings/errors like this one:

  CC      ppc64-softmmu/hw/intc/xics.o
In file included from hw/intc/xics.c:35:
include/hw/ppc/xics.h:43:25: error: redefinition of typedef 'ICPState' is a C11 feature
      [-Werror,-Wtypedef-redefinition]
typedef struct ICPState ICPState;
                        ^
target/ppc/cpu.h:1181:25: note: previous definition is here
typedef struct ICPState ICPState;
                        ^
Work around the problems by including the proper headers in spapr.h
and by using struct forward declarations in cpu.h.

Reviewed-by: Greg Kurz <groug@kaod.org>
Signed-off-by: Thomas Huth <thuth@redhat.com>