target/arm/cpregs: Include missing 'target/arm/cpu.h' header

CPReadFn type definitions use the CPUARMState type, itself
declared in "cpu.h". Include this file in order to avoid when
refactoring headers:

  ../target/arm/cpregs.h:241:27: error: unknown type name 'CPUARMState'
  typedef uint64_t CPReadFn(CPUARMState *env, const ARMCPRegInfo *opaque);
                            ^

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-id: 20250513173928.77376-5-philmd@linaro.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>