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>

Author: stffrdhrn

arch/openrisc/kernel/signal.c

@@ -229,7 +229,7 @@ handle_signal(struct ksignal *ksig, struct pt_regs *regs)
  * mode below.
  */
 
-int do_signal(struct pt_regs *regs, int syscall)
+static int do_signal(struct pt_regs *regs, int syscall)
 {
 	struct ksignal ksig;
 	unsigned long continue_addr = 0;