raid6: fix raid6_recov_rvv symbol undeclared warning

raid6: fix raid6_recov_rvv symbol undeclared warning

The riscv recov_rvv.c should have included pq_arch.h for
the definition of raid6_recov_rvv. Add the include to
fix the following sparse warning:

lib/raid/raid6/riscv/recov_rvv.c:218:32: warning: symbol 'raid6_recov_rvv' 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/20260622135535.481534-1-ben.dooks@codethink.co.uk
Signed-off-by: Paul Walmsley <pjw@kernel.org>