win32: Don't use dllexport/import when only building statically

win32: Don't use dllexport/import when only building statically

If the prototypes in the public API have dllimport in them when building
statically on Windows, the compiler will look for symbols with symbol
mangling and indirection corresponding to a DLL. This will cause a build
failure when trying to link tests/examples/etc.

External users of GStreamer also need to define -DGST_STATIC_COMPILATION
if they want to link to static gstreamer libraries on Windows.

A similar version of this patch has been committed to all gstreamer
repositories.

https://bugzilla.gnome.org/show_bug.cgi?id=767463