x86/tlb/uv: Add a forward declaration for struct flush_tlb_info

... to fix these build warnings:

  In file included from ./arch/x86/include/asm/uv/uv_hub.h:22,
                   from drivers/misc/sgi-gru/grukdump.c:16:
  ./arch/x86/include/asm/uv/uv.h:39:21: warning: ‘struct flush_tlb_info’ declared \
     inside parameter list will not be visible outside of this definition or declaration
     39 |        const struct flush_tlb_info *info);
        |                     ^~~~~~~~~~~~~~
  In file included from ./arch/x86/include/asm/uv/uv_hub.h:22,
                   from drivers/misc/sgi-gru/grutlbpurge.c:28:
  ./arch/x86/include/asm/uv/uv.h:39:21: warning: ‘struct flush_tlb_info’ declared \
    inside parameter list will not be visible outside of this definition or declaration
     39 |        const struct flush_tlb_info *info);
        |                     ^~~~~~~~~~~~~~

  ...

after

  bfe3d8f6 ("x86/tlb: Restrict access to tlbstate")

restricted access to tlbstate.

Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20200503103107.3419-1-bp@alien8.de