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>

arch/riscv/Makefile

@@ -77,4 +77,8 @@ core-y += arch/riscv/kernel/ arch/riscv/mm/
 
 libs-y += arch/riscv/lib/
 
+PHONY += vdso_install
+vdso_install:
+	$(Q)$(MAKE) $(build)=arch/riscv/kernel/vdso $@
+
 all: vmlinux