raid6: fix riscv symbol undeclared warnigns

raid6: fix riscv symbol undeclared warnigns

The riscv rvv.c file is missing the include of pq_arch.h
which defines all the exported functions. Include this
to remove the following sparse warnings:

lib/raid/raid6/riscv/rvv.c:1225:1: warning: symbol 'raid6_rvvx1' was not declared. Should it be static?
lib/raid/raid6/riscv/rvv.c:1226:1: warning: symbol 'raid6_rvvx2' was not declared. Should it be static?
lib/raid/raid6/riscv/rvv.c:1227:1: warning: symbol 'raid6_rvvx4' was not declared. Should it be static?
lib/raid/raid6/riscv/rvv.c:1228:1: warning: symbol 'raid6_rvvx8' was not declared. Should it be static?

Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Reviewed-by: Nam Cao <namcao@linutronix.de>
Link: https://patch.msgid.link/20260622135207.480540-1-ben.dooks@codethink.co.uk
Signed-off-by: Paul Walmsley <pjw@kernel.org>