ignored containers are part of a shared backend code
ignored_containers_sdn is defined in src/back-shr.c and referenced in
src/back-shr.h for the actual backends to use. Header file, however, has
to use 'external' or it would be defining the variable at each
inclusion.
Fixes: RHBZ#1800097
Signed-off-by: Alexander Bokovoy <abokovoy@redhat.com>