Exclude sync() usage from VxWorks platform

This function is not available in VxWorks libraries and it is not
mandatory for proper qstorageinfo test execution.

Task-number: QTBUG-115777
Change-Id: Ic503c776002087d461c32f890795c27be4b1ca34
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
(cherry picked from commit c0c66a42ced590cadfb55efc82f80af5f035020e)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

tests/auto/corelib/io/qstorageinfo/tst_qstorageinfo.cpp

@@ -255,7 +255,9 @@ void tst_QStorageInfo::freeSpaceUpdate()
         FlushFileBuffers(HANDLE(_get_osfhandle(file.handle())));
 #elif _POSIX_VERSION >= 200112L
         fsync(file.handle());
+# ifndef Q_OS_VXWORKS
         sync();
+# endif // Q_OS_VXWORKS
 #endif
     };