Move Emscripten includes to top of `main.cpp`

All system header includes should come before project-specific includes
(includes specific to this game), while coming after the include
specific to the given file (if any; main.cpp doesn't have any).

desktop_version/src/main.cpp

@@ -1,4 +1,8 @@
 #include <SDL.h>
+#ifdef __EMSCRIPTEN__
+#include <emscripten.h>
+#include <emscripten/html5.h>
+#endif
 
 #include "CustomLevels.h"
 #include "DeferCallbacks.h"
@@ -24,11 +28,6 @@
 #include "UtilityClass.h"
 #include "Vlogging.h"
 
-#ifdef __EMSCRIPTEN__
-#include <emscripten.h>
-#include <emscripten/html5.h>
-#endif
-
 scriptclass script;
 
 #ifndef NO_CUSTOM_LEVELS