Fixes needed for me to compile qemu with nios2 support with GCC 4.3.3

The typedef is already done after the forward declaration. This leads to
the following error when compiling with GCC 4.4.3:

In file included from /home/tklauser/src/mb-qemu-labx/qemu-common.h:128,
                 from /home/tklauser/src/mb-qemu-labx/exec.c:27:
/home/tklauser/src/mb-qemu-labx/target-nios2/cpu.h:158: error: redefinition of typedef ‘CPUNios2State’
/home/tklauser/src/mb-qemu-labx/target-nios2/cpu.h:33: note: previous declaration of ‘CPUNios2State’ was here

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>