KVM: selftests: Remove errant asm/barrier.h include to fix arm64 build

Drop an unnecessary include of asm/barrier.h from dirty_log_test.c to
allow the test to build on arm64.  arm64, s390, and x86 all build cleanly
without the include (PPC and MIPS aren't supported in KVM's selftests).

arm64's barrier.h includes linux/kasan-checks.h, which is not copied
into tools/.

  In file included from ../../../../tools/include/asm/barrier.h:8,
                   from dirty_log_test.c:19:
     .../arm64/include/asm/barrier.h:12:10: fatal error: linux/kasan-checks.h: No such file or directory
     12 | #include <linux/kasan-checks.h>
        |          ^~~~~~~~~~~~~~~~~~~~~~
  compilation terminated.

Fixes: 84292e56

 ("KVM: selftests: Add dirty ring buffer test")
Cc: Peter Xu <peterx@redhat.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20210622200529.3650424-2-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>