riscv: vdso: include vdso/vsyscall.h for vdso_data

Add include of <vdso/vsyscall.h> to pull in the defition of vdso_data
to remove the following sparse warning:

arch/riscv/kernel/vdso.c:39:18: warning: symbol 'vdso_data' was not declared. Should it be static?

Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Link: https://lore.kernel.org/r/20230616114357.159601-1-ben.dooks@codethink.co.uk
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>