Move SecureString class to ipmid/types.hpp

SecureString class doesn't have access in user_library to use in
other files

Tested:
Added class to ipmid/types.hpp from user_channel/user_mgmt.hpp
Build got successful.

Signed-off-by: Snehalatha Venkatesh <snehalathax.v@intel.com>
Change-Id: I5e0c4bb0744113e70540b272f16a5116421048fb
3 files changed