gstconfig: Use __declspec when built with MinGW and linking with MSVC

gstconfig: Use __declspec when built with MinGW and linking with MSVC

Earlier we were only using __declspec(dllexport/import) when we were
built with MSVC because when built with MinGW and linking with MinGW we
don't need it (and we get linker errors because of it).

However, when we're built with MinGW and someone wants to link to us
with MSVC, we still need the prototypes to have __declspec(dllimport)
since MSVC cannot do auto-import like GCC can.

https://bugzilla.gnome.org/show_bug.cgi?id=771029