microblaze: signal: Declare do_notify_resume

Fix sparse warning by declaration do_notify_resume function
called from entry.S.
Warning:
arch/microblaze/kernel/signal.c:357:6: warning: symbol
'do_notify_resume' was not declared. Should it be static?

Signed-off-by: Michal Simek <michal.simek@xilinx.com>