mgmt/mcumgr/lib: Add missing inclusion of toolchain.h

The zcbor_bulk_priv.h uses STRINGIFY for some of definitions
and, due to lack of inclusion of the toolchain.h, that was causing
compilation errors when NEWLIB would be selected.

Fixes #44811.

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>