Add DLL import/export directive

[rickyviking]

I think it might be useful to add DLL import directive before the classes defined by the project, to make them ready to use when the OsgOculus library is built as DLL and used in other DLLs/executables.

If you agree, I can propose a stub through a PR.

[bjornblissing]

Sure, send me a PR with a stub.

[rickyviking]

Ok, see PR https://github.com/bjornblissing/osgoculusviewer/pull/108 The oculusconfig.h file is included in "" quotes, to make it easier to include the library in bigger project using a subdirectory