libeventc: Update VAPI

Signed-off-by: Quentin Glidic <sardemff7+git@sardemff7.net>
libeventc: Add connect_sync()

Signed-off-by: Quentin Glidic <sardemff7+git@sardemff7.net>
eventc: Use eventc_connection_connect_sync()

Signed-off-by: Quentin Glidic <sardemff7+git@sardemff7.net>