Add export pragmas to smt.h

[CyanoKobalamyne]

This stops clangd from complaining about unused includes.

[CyanoKobalamyne]

Also, if the intention is for users to not include the individual headers referenced in smt.h, then we should probably mark them as private.

https://clangd.llvm.org/design/include-cleaner#umbrella-headers

[yoni206]

Can we instead suppress related warnings through cmake?
If we can, then perhaps this would be more sustainable (the next person that adds an #include might forget to add these pragmas).
What do you think?

[barrettcw]

Actually, I would be in favor of using these pragmas and also documenting each "internal" header file to say that this file should not be included directly by a user. I think this seems like a better practice than disabling the warnings. If you're worried someone will forget in the future, we could also add some text in a "developer's wiki" explaining this policy.