Replace header guards with #pragma once

Signed-off-by: Leon Marz <main@lmarz.org>

src/LyricsWindow.h

@@ -9,8 +9,7 @@
 
 **********************************************************************/
 
-#ifndef __AUDACITY_LYRICS_WINDOW__
-#define __AUDACITY_LYRICS_WINDOW__
+#pragma once
 
 #include <wx/frame.h> // to inherit
 
@@ -46,5 +45,3 @@ class LyricsWindow final : public wxFrame,
  public:
    DECLARE_EVENT_TABLE()
 };
-
-#endif