um: port_kern: fix -Wmissing-variable-declarations

um: port_kern: fix -Wmissing-variable-declarations

I'm looking to enable -Wmissing-variable-declarations behind W=1. 0day
bot spotted the following instance:

  arch/um/drivers/port_kern.c:147:14: warning: no previous extern
  declaration for non-static variable 'port_work'
  [-Wmissing-variable-declarations]
  DECLARE_WORK(port_work, port_work_proc);
               ^
  arch/um/drivers/port_kern.c:147:1: note: declare 'static' if the
  variable is not intended to be used outside of this translation unit
  DECLARE_WORK(port_work, port_work_proc);
  ^

This symbol is not referenced by more than one translation unit, so give
it static storage.

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/llvm/202308081050.sZEw4cQ5-lkp@intel.com/
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Richard Weinberger <richard@nod.at>