x86/platform/uv: Add a missing prototype for uv_bau_message_interrupt()

... in order to fix a -Wmissing-prototypes warning:

  arch/x86/platform/uv/tlb_uv.c:1275:6: warning:
  no previous prototype for ‘uv_bau_message_interrupt’ [-Wmissing-prototypes] \
	  void uv_bau_message_interrupt(struct pt_regs *regs)

Signed-off-by: Benjamin Thiel <b.thiel@posteo.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20200327072621.2255-1-b.thiel@posteo.de