openrisc: Add missing prototypes for assembly called fnctions

These functions are all called from assembly files so there is no need
for a prototype in a header file, but when compiling with W=1 enabling
-Wmissing-prototypes the compiler warns:

arch/openrisc/kernel/ptrace.c:191:17: error: no previous prototype for 'do_syscall_trace_enter' [-Werror=missing-prototypes]
arch/openrisc/kernel/ptrace.c:210:17: error: no previous prototype for 'do_syscall_trace_leave' [-Werror=missing-prototypes]
arch/openrisc/kernel/signal.c:293:1: error: no previous prototype for 'do_work_pending' [-Werror=missing-prototypes]
arch/openrisc/kernel/signal.c:68:17: error: no previous prototype for '_sys_rt_sigreturn' [-Werror=missing-prototypes]
arch/openrisc/kernel/time.c:111:25: error: no previous prototype for 'timer_interrupt' [-Werror=missing-prototypes]
arch/openrisc/kernel/traps.c:239:17: error: no previous prototype for 'unhandled_exception' [-Werror=missing-prototypes]
arch/openrisc/kernel/traps.c:246:17: error: no previous prototype for 'do_fpe_trap' [-Werror=missing-prototypes]
arch/openrisc/kernel/traps.c:268:17: error: no previous prototype for 'do_trap' [-Werror=missing-prototypes]
arch/openrisc/kernel/traps.c:273:17: error: no previous prototype for 'do_unaligned_access' [-Werror=missing-prototypes]
arch/openrisc/kernel/traps.c:286:17: error: no previous prototype for 'do_bus_fault' [-Werror=missing-prototypes]
arch/openrisc/kernel/traps.c:462:17: error: no previous prototype for 'do_illegal_instruction' [-Werror=missing-prototypes]
arch/openrisc/mm/fault.c:44:17: error: no previous prototype for 'do_page_fault' [-Werror=missing-prototypes]

Since these are not needed in header files, fix these by adding
prototypes to the top of the respective C files.

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>