tests/mgmt/mcumgr/smp_reassembly: Rename mgmt_hdr to smp_hdr

SMP reassembly is the only non-protocol code in MCUmgr that
directly accesses SMP header; this tests has been accessing
mgmt_hdr structure and now it is named smp_hdr and visible
via smp_internal.h.

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