include: refuse inclusion of seperate header files

include: refuse inclusion of seperate header files

Later on it might make sense to move around function definitions between
header files. Hence make sure that nobody includes any header file
directly except the main one.

Signed-off-by: Lennart Poettering <lennart@poettering.net>
Reviewed-By: Jan Safranek <jsafrane@redhat.com>
Acked-by: Dhaval Giani <dhaval.giani@gmail.com>
Signed-off-by: Dhaval Giani <dhaval.giani@gmail.com>