KVM: Declare kvm_arch_flush_remote_tlbs() globally

There's no reason for the architectures to declare
kvm_arch_flush_remote_tlbs() in their own headers. Hence to
avoid this duplication, make the declaration global, leaving
the architectures to define only __KVM_HAVE_ARCH_FLUSH_REMOTE_TLBS
as needed.

Signed-off-by: Raghavendra Rao Ananta <rananta@google.com>
Reviewed-by: Shaoqin Huang <shahuang@redhat.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20230811045127.3308641-3-rananta@google.com