gstconfig.h: Always use dllexport/import on Windows

gstconfig.h: Always use dllexport/import on Windows

__declspec(dllexport/import) are supported by GCC and are needed for
properly generating code that fetches the values of constants from DLLs
built with __declspec(dllexport) which happens when anything using
GST_EXPORT is built with MSVC.

See: https://msdn.microsoft.com/en-us/library/619w14ds.aspx

Essentially, if you built gstreamer with MSVC and then tried to use
constants from it (such as GST_TYPE_CAPS) in a plugin, GCC would
retrieve the address of the value instead of the value itself.