UICommon/NetPlayIndex: Use std::move within SetErrorCallback()

std::function is allowed to heap allocate in order to store captured
variables, etc, so std::function isn't a trivial type. We can std::move
here in order to avoid potential reallocating.

While we're at it, make the definition's parameter name match the
declaration's parameter name for consistency.

Source/Core/UICommon/NetPlayIndex.cpp

@@ -333,7 +333,7 @@ bool NetPlayIndex::HasActiveSession() const
   return !m_secret.empty();
 }
 
-void NetPlayIndex::SetErrorCallback(std::function<void()> function)
+void NetPlayIndex::SetErrorCallback(std::function<void()> callback)
 {
-  m_error_callback = function;
+  m_error_callback = std::move(callback);
 }