mds/Migrator.h: init some export_state_t members in ctor

CID 1188165 (#1 of 1): Uninitialized scalar field (UNINIT_CTOR)
 2. uninit_member: Non-static class member "state" is not initialized in
  this constructor nor in any functions that it calls.
 4. uninit_member: Non-static class member "peer" is not initialized in this
  constructor nor in any functions that it calls.
 6. uninit_member: Non-static class member "tid" is not initialized in this
  constructor nor in any functions that it calls.

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>