server: move include directives before extern "C" wrapper

Analogous to previous commit but for the server(-core) header.

Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Quentin Glidic <sardemff7+git@sardemff7.net>