Added SDL_GetDefaultKeyFromScancode() to the public API

This is useful when getting the English name of a key for a non-Latin keyboard layout, for example.

src/dynapi/SDL_dynapi.sym

@@ -209,6 +209,7 @@ SDL3_0.0.0 {
     SDL_GetDaysInMonth;
     SDL_GetDefaultAssertionHandler;
     SDL_GetDefaultCursor;
+    SDL_GetDefaultKeyFromScancode;
     SDL_GetDesktopDisplayMode;
     SDL_GetDisplayBounds;
     SDL_GetDisplayContentScale;