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 <[e-mail]>
Closes: https://lore.kernel.org/llvm/[e-mail]/
Signed-off-by: Nick Desaulniers <[e-mail]>
Signed-off-by: Richard Weinberger <[e-mail]>

arch/um/drivers/port_kern.c

···
 	local_irq_restore(flags);
 }
 
-DECLARE_WORK(port_work, port_work_proc);
+static DECLARE_WORK(port_work, port_work_proc);
 
 static irqreturn_t port_interrupt(int irq, void *data)
 {