mds/Mutation.h: init export_dir with NULL in ctor

CID 1188167 (#1 of 1): Uninitialized pointer field (UNINIT_CTOR)
 2. uninit_member: Non-static class member "export_dir" is not initialized in
 this constructor nor in any functions that it calls.

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