libs: fix API export/import and 'inconsistent linkage' on MSVC

libs: fix API export/import and 'inconsistent linkage' on MSVC

Export rtsp-server library API in headers when we're building the
library itself, otherwise import the API from the headers.

This fixes linker warnings on Windows when building with MSVC.

Fix up some missing config.h includes when building the lib which
is needed to get the export api define from config.h

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