riscv: fix building external modules

When building external modules, vdso_prepare should not be run.  If the
kernel sources are read-only, it will fail.

Fixes: fde9c59a

 ("riscv: explicitly use symbol offsets for VDSO")
Signed-off-by: Andreas Schwab <schwab@suse.de>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Tested-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>