accel/kvm: Make kvm_irqchip*notifier() declaration non target-specific

Commit 3607715a308 ("kvm: Introduce KVM irqchip change notifier")
restricted the kvm_irqchip*notifier() declarations to target-specific
files, guarding them under the NEED_CPU_H (later renamed as
COMPILING_PER_TARGET) #ifdef check.

This however prohibit building the kvm-stub.c file once:

  ../accel/stubs/kvm-stub.c:70:6: error: no previous prototype for function 'kvm_irqchip_add_change_notifier' [-Werror,-Wmissing-prototypes]
     70 | void kvm_irqchip_add_change_notifier(Notifier *n)
        |      ^
  ../accel/stubs/kvm-stub.c:74:6: error: no previous prototype for function 'kvm_irqchip_remove_change_notifier' [-Werror,-Wmissing-prototypes]
     74 | void kvm_irqchip_remove_change_notifier(Notifier *n)
        |      ^
  ../accel/stubs/kvm-stub.c:78:6: error: no previous prototype for function 'kvm_irqchip_change_notify' [-Werror,-Wmissing-prototypes]
     78 | void kvm_irqchip_change_notify(void)
        |      ^

Since nothing in these prototype declarations is target specific,
move them around to be generically available, allowing to build
kvm-stub.c once for all targets in the next commit.

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
Message-Id: <20260309174941.67624-2-philmd@linaro.org>