ceph_test_objectstore: fix i386 build (again)

test/objectstore/store_test.cc: In member function ‘void SyntheticWorkloadState::read()’:
error: test/objectstore/store_test.cc:462:23: no matching function for call to ‘swap(uint64_t&, size_t&)’

Signed-off-by: Sage Weil <sage@inktank.com>