gnu: Transmission: Fix localization of transmission-gtk.

gnu: Transmission: Fix localization of transmission-gtk.

* gnu/packages/bittorrent.scm (transmission)[source]: Add patch to make
Transmission honor the "localedir" option to its configure script.
[arguments]<configure-flags>: Use "localedir" to set the "gui" output as the
location for (GTK-specific) localization data.
<phases>: Update "move-gui" to remove a now-superfluous call to "mkdir" and omit
"share/locale" from the list of directories needing to be moved between outputs.
* gnu/packages/patches/transmission-honor-localedir.patch: New file.
* gnu/local.mk (dist_patch_DATA): Add it.

Signed-off-by: Leo Famulari <leo@famulari.name>