target: Include missing 'cpu.h'

target: Include missing 'cpu.h'

These target-specific files use the target-specific CPU state
but lack to include "cpu.h"; i.e.:

    ../target/riscv/pmp.h:61:23: error: unknown type name 'CPURISCVState'
    void pmpcfg_csr_write(CPURISCVState *env, uint32_t reg_index,
                          ^
    ../target/nios2/mmu.h:43:18: error: unknown type name 'CPUNios2State'
    void mmu_flip_um(CPUNios2State *env, unsigned int um);
                     ^
    ../target/microblaze/mmu.h:88:19: error: unknown type name 'CPUMBState'; did you mean 'CPUState'?
    uint32_t mmu_read(CPUMBState *env, bool ea, uint32_t rn);
                      ^~~~~~~~~~
                      CPUState

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20220214183144.27402-10-f4bug@amsat.org>