RISC-V: Fix do_notify_resume / do_work_pending prototype

Commit b0f4c74eadbf ("RISC-V: Fix unannoted hardirqs-on in return to
userspace slow-path") renamed the do_notify_resume function to
do_work_pending but did not change the prototype in signal.h
Do that now, as the original function does not exist anymore.

Fixes: b0f4c74eadbf ("RISC-V: Fix unannoted hardirqs-on in return to userspace slow-path")
Signed-off-by: Heiko Stuebner <heiko.stuebner@vrull.eu>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20230118142252.337103-1-heiko@sntech.de
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>

riscv: signal: fix missing prototype warning

Fix the warning:
arch/riscv/kernel/signal.c:316:27: warning: no previous prototype for function 'do_notify_resume' [-Wmissing-prototypes]
asmlinkage __visible void do_notify_resume(struct pt_regs *regs,

All other functions in the file are static & none of the existing
headers stood out as an obvious location. Create signal.h to hold the
declaration.

Fixes: e2c0cdfba7f6 ("RISC-V: User-facing API")
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20220814141237.493457-4-mail@conchuod.ie
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>