PEL: Print Failing MTMS section into JSON

Remove any trailing whitespace for serial number.

"Failing MTMS": {
    "Section Version":          "1",
    "Sub-section type":         "0",
    "Created by":               "0x4552",
    "Machine Type Model":       "OPWR-131",
    "Serial Number":            "1318ABA"
},

Testing: Manually run peltool and verified output. All unit tests
passed.
Signed-off-by: Harisuddin Mohamed Isa <harisuddin@gmail.com>
Change-Id: I7a05790bdf406ef6d7946ffae831a45ee6dd5cda
diff --git a/extensions/openpower-pels/failing_mtms.hpp b/extensions/openpower-pels/failing_mtms.hpp
index 65186eb..8f5cd44 100644
--- a/extensions/openpower-pels/failing_mtms.hpp
+++ b/extensions/openpower-pels/failing_mtms.hpp
@@ -81,6 +81,12 @@
         return _mtms.machineSerialNumber();
     }
 
+    /**
+     * @brief Get section in JSON.
+     * @return std::optional<std::string> - Failing MTMS section in JSON
+     */
+    std::optional<std::string> getJSON() const override;
+
   private:
     /**
      * @brief Validates the section contents