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
diff --git a/user_channel/user_mgmt.cpp b/user_channel/user_mgmt.cpp
index 036f73d..6b31eb9 100644
--- a/user_channel/user_mgmt.cpp
+++ b/user_channel/user_mgmt.cpp
@@ -27,6 +27,7 @@
 #include <boost/interprocess/sync/scoped_lock.hpp>
 #include <cerrno>
 #include <fstream>
+#include <ipmid/types.hpp>
 #include <nlohmann/json.hpp>
 #include <phosphor-logging/elog-errors.hpp>
 #include <phosphor-logging/log.hpp>