net: bonding: Add SPDX identifier to remaining files

Previous batches of SPDX conversion missed bond_main.c and bonding_priv.h
because these files doesn't mention intended GPL version. Add SPDX identifier
to these files, assuming GPL 1.0+.

Cc: Thomas Davis <tadavis@lbl.gov>
Cc: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Cc: Stephen Hemminger <stephen@networkplumber.org>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: Bagas Sanjaya <bagasdotme@gmail.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>