riscv: add missing vdso_install target

[ Upstream commit f157d411a9eb170d2ee6b766da7a381962017cc9 ]

Building kernel 4.20 for Fedora as RPM fails, because riscv is missing
vdso_install target in arch/riscv/Makefile.

Signed-off-by: David Abdurachmanov <david.abdurachmanov@gmail.com>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>