LibCore: Add ElapsedTimer::reset()

Author: https://github.com/gmta
Commit: 7c939c58b8
Pull-request: https://github.com/SerenityOS/serenity/pull/10662
Reviewed-by: https://github.com/awesomekling

Userland/Libraries/LibCore/ElapsedTimer.h

@@ -22,6 +22,7 @@ public:
 
     bool is_valid() const { return m_valid; }
     void start();
+    void reset();
     int elapsed() const;
     Time elapsed_time() const;