Make powerline-symbols unconditional dependency

Reasoning was described in Lokaltog/powerline#719.

---

By the way, do you know why

1. there are no Manifest files and
2. ebuild refuses to create them with the following log:

    ```
    (zyx-desktop:zyx:tmp/image/raiagent) 1 % ebuild app-misc/powerline/powerline-9999.ebuild manifest
    !!! Repository 'mv' is missing masters attribute in '/var/lib/layman/mv/metadata/layout.conf'
    !!! Set 'masters = gentoo' in this file for future compatibility                                                                                                                                                                               
    !!! Repository 'enlightenment' is missing masters attribute in '/var/lib/layman/enlightenment/metadata/layout.conf'                                                                                                                            
    !!! Set 'masters = gentoo' in this file for future compatibility                                                                                                                                                                               
    Appending /home/zyx/tmp/image/raiagent to PORTDIR_OVERLAY...
    !!! Repository 'mv' is missing masters attribute in '/var/lib/layman/mv/metadata/layout.conf'
    !!! Set 'masters = gentoo' in this file for future compatibility                                                                                                                                                                               
    !!! Repository 'enlightenment' is missing masters attribute in '/var/lib/layman/enlightenment/metadata/layout.conf'                                                                                                                            
    !!! Set 'masters = gentoo' in this file for future compatibility                                                                                                                                                                               
    >>> Creating Manifest for /home/zyx/tmp/image/raiagent/app-misc/powerline
    ```

    which looks like if it actually created Manifest.