openrisc: Declare do_signal function as static

When compiling with W=1 enabling -Wmissing-prototypes the compiler
warns:

  arch/openrisc/kernel/signal.c:227:5: error: no previous prototype for 'do_signal' [-Werror=missing-prototypes]

Fix this by declaring the function a static as it is not used outside of
the scope of this file.

Reported-by: Arnd Bergmann <arnd@arndb.de>
Closes: https://lore.kernel.org/linux-kernel/20230810141947.1236730-17-arnd@kernel.org/


Signed-off-by: Stafford Horne <shorne@gmail.com>