oemcommands: use SecureString for password

The ipmiSetSpecialUserPassword API changed to now require a SecureString
instead of a std::string.

See phosphor-host-ipmid commit 1e22a0f1a0348846eb56f534f101776dc445b764.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: Idc7a7f80b299b0753c958ae6d012812f2081810e
diff --git a/src/oemcommands.cpp b/src/oemcommands.cpp
index 3cd26bc..9fb4892 100644
--- a/src/oemcommands.cpp
+++ b/src/oemcommands.cpp
@@ -1241,7 +1241,7 @@
     if (userPassword.size() != 0)
     {
         constexpr uint8_t minPasswordSizeRequired = 6;
-        std::string passwd;
+        SecureString passwd;
         if (userPassword.size() < minPasswordSizeRequired ||
             userPassword.size() > ipmi::maxIpmi20PasswordSize)
         {