arm64: Remove unnecessary irqflags alternative.h include

Since commit 20af807d ("arm64: Avoid cpus_have_const_cap() for
ARM64_HAS_GIC_PRIO_MASKING"), the alternative.h include is not used,
so remove it.

Fixes: 20af807d

 ("arm64: Avoid cpus_have_const_cap() for ARM64_HAS_GIC_PRIO_MASKING")
Signed-off-by: Jinjie Ruan <ruanjinjie@huawei.com>
Link: https://lore.kernel.org/r/20240314063819.2636445-1-ruanjinjie@huawei.com
Signed-off-by: Will Deacon <will@kernel.org>