target/riscv/cpu.c: fix Zvkb extension config

target/riscv/cpu.c: fix Zvkb extension config

This code has a typo that writes zvkb to zvkg, causing users can't
enable zvkb through the config. This patch gets this fixed.

Signed-off-by: Yangyu Chen <cyy@cyyself.name>
Fixes: ea61ef7097d0 ("target/riscv: Move vector crypto extensions to riscv_cpu_extensions")
Reviewed-by: LIU Zhiwei <zhiwei_liu@linux.alibaba.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Max Chou <max.chou@sifive.com>
Reviewed-by:  Weiwei Li <liwei1518@gmail.com>
Message-ID: <tencent_7E34EEF0F90B9A68BF38BEE09EC6D4877C0A@qq.com>
Cc: qemu-stable <qemu-stable@nongnu.org>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>