Add workaround for obscure linker error.
Add workaround for obscure linker error.

The "dub test" on the root package failed with:
appmain.d:_D49TypeInfo_S3std8typecons15__T8NullableTiZ8Nullable6__initZ: error: undefined reference to '_D3std8typecons15__T8NullableTiZ8Nullable6__initZ'

gedaiu/vibe.d,rejectedsoftware/vibe.d,schuetzm/vibe.d,Geod24/vibe.d,chalucha/vibe.d,AndreyMZ/vibe.d,etcimon/vibe.d,Mihail-K/vibe.d,etcimon/vibe.d,DBankov/vibe.d