[SimToSV] Add include guards to DPI import

[uenoku]

This adds include guards __CIRCT_DPI_IMPORT_* to DPI import statements generated in SimToSV.

Fix #7458.

[fabianschuiki]

LGTM!

It's pretty convenient that the sv.func.dpi.import op is just a marker that points at a function declaration somewhere else. Moving the actual function with the actual @... symbol name into would probably have been a lot more annoying, right?

[uenoku]

Moving the actual function with the actual @... symbol name into would probably have been a lot more annoying, right?

Yes the function declaration must be a child of SymbolTable op and we cannot nest that.