version.hpp: Pass const reference

In 1fd6ddd it adds a std::string& as parameter, it is better to use
const std::string&.
In the following commit, there is a change that requires const
reference.

Signed-off-by: Lei YU <yulei.sh@bytedance.com>
Change-Id: Ie3f350e577d944dd4d7e65a4790b2fead3d7eecb
diff --git a/version.hpp b/version.hpp
index a93709f..8a68cb5 100644
--- a/version.hpp
+++ b/version.hpp
@@ -78,7 +78,7 @@
      */
     Version(sdbusplus::bus::bus& bus, const std::string& objPath,
             const std::string& versionString, VersionPurpose versionPurpose,
-            std::string& extVersion, const std::string& filePath,
+            const std::string& extVersion, const std::string& filePath,
             eraseFunc callback) :
         VersionInherit(bus, (objPath).c_str(), true),
         eraseCallback(callback), versionStr(versionString)