riscv: add missing vdso_install target

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>