Theme symlink fix

When using a symlink to another directory for custom themes (e.g. to avoid having it in /usr/share/otrs/ which should only contain files contained in packages), using relative paths containing "../" will refer to files in the parent of the linked directory and not the OTRS themes directory. Therefore one should avoid relative paths with "../".

Without this patch symlinking a theme will cause a lot of these error messages:

    Message: Can't read /usr/share/otrs/Kernel/Output/HTML/SomeTheme/../Standard/Header.dtl: No such file or directory