[Hotfix] Mark python-FFI handling with TVM_DLL

[Lunderberg]

Bugfix for builds on Windows, which has missing symbol exports.

[tqchen]

you want to place declaration in the header like the others and put TVM_DLL there. TVM_DLL do not play well with inline extern C and need to work with

extern "C"{
TVM_DLL func
}

[junrushao]

We will further need to expose those methods in a .h file, wrapped with extern "C", and mark with TVM_DLL :-( MSVC is weird

[Lunderberg]

Whoops, looks like renaming the branch on github accidentally closed the PR.

Thank you on the MSVC weirdnesses, and I've updated the PR to match the way all other C API functions are exposed. (And took it as an opportunity to add documentation on what I had previously considered internal implementation functions.)

[junrushao]

I'd love to get this PR in quickly as well as a hotfix, but seems i'm unable to get around the CI...

[Lunderberg]

I'd love to get this PR in quickly as well as a hotfix, but seems i'm unable to get around the CI...

I think there's a skip CI flag, though whether it is [skipci], [Skip_CI], or some other value, and whether I'd need to have had it present when the PR was initially submitted, I'm not sure.