common: move private headers from include/common to common

Signed-off-by: Tim Blechmann <tim@klingt.org>
plugin interface: move struct SC_Lock out of the public interface

plugins only acquire the NRT lock, but should to that via the interface
table

Signed-off-by: Tim Blechmann <tim@klingt.org>
scapp: add include dirs for private headers

Signed-off-by: Tim Blechmann <tim@klingt.org>