riscv: Fix warning in riscv_regno_ok_for_index_p

riscv: Fix warning in riscv_regno_ok_for_index_p

The variable `regno` is currently not used in riscv_regno_ok_for_index_p(),
which triggers a compiler warning. Let's address this.

Fixes: 423604278ed5 ("riscv: Prepare backend for index registers")
Reported-by: Juzhe Zhong <juzhe.zhong@rivai.ai>
Reported-by: Andreas Schwab <schwab@linux-m68k.org>
Signed-off-by: Christoph Müllner <christoph.muellner@vrull.eu>
gcc/ChangeLog:

	* config/riscv/riscv.cc (riscv_regno_ok_for_index_p):
	Remove parameter name from declaration of unused parameter.

Signed-off-by: Christoph Müllner <christoph.muellner@vrull.eu>