Add DragonFly to WTF_OS_FREEBSD

DragonFly need to be considered as FREEBSD to avoid compile errors.

Change Line 398 of qtscript/src/3rdparty/javascriptcore/JavaScriptCore/wtf/Platform.h, it'll be something like these.

#if defined(__FreeBSD__) || defined(__DragonFly__)
#define WTF_OS_FREEBSD 1
#endif

After this little modification, qtscript would be built successfully.