gstconfig.h: add GST_API_IMPORT define

gstconfig.h: add GST_API_IMPORT define

This is for use by the various GST_*_API decorators and
will be what they get defined to when a library API is being
used by external users of that library (not the library itself
whilst it's being compiled).

In most cases it will simply map to a plain 'extern' but on
Windows with MSVC it will need to map to __declspec(dllimport).
For functions this is not strictly needed, but for exported
variables it is.

https://bugzilla.gnome.org/show_bug.cgi?id=797185