nds32: Fix compiler warning, Wstringop-overflow, in vdso.c

Getting a compiler warning, Wstringop-overflow, in
arch/nds32/kernel/vdso.c when kernel is built by gcc-8. Declaring
vdso_start and vdso_end as a pointer to fix this compiler warning.

Signed-off-by: Vincent Chen <vincentc@andestech.com>
Reviewed-by: Greentime Hu <greentime@andestech.com>
Signed-off-by: Greentime Hu <greentime@andestech.com>