test: osd/TestOSDMap.cc: fix Clang complain about promotion

test: osd/TestOSDMap.cc: fix Clang complain about promotion

/home/jenkins/workspace/ceph-freebsd/src/test/osd/TestOSDMap.cc:444:15: error: non-constant-expression cannot be narrowed from type 'unsigned int' to 'int' in initializer list [-Wc++11-narrowing]
    m.set(x, {i});
              ^
/home/jenkins/workspace/ceph-freebsd/src/test/osd/TestOSDMap.cc:444:15: note: insert an explicit cast to silence this issue
    m.set(x, {i});
              ^
              static_cast<int>( )
1 error generated.

Signed-off-by: Willem Jan Withagen <wjw@digiware.nl>