Fix the path for loading solarized.

Once all extensions were moved from "extensions" to "local",
spacemacs was no longer able to find the solarized themes.

Thanks to @TheBB for helping narrow this down!

Closes syl20bnr#2628

core/core-themes-support.el

@@ -183,7 +183,7 @@ package name does not match theme name + `-theme' suffix.")
      ((or (eq 'solarized-light theme)
           (eq 'solarized-dark theme))
       (add-to-list 'load-path (concat spacemacs-directory
-                                      "extensions/solarized-theme/"))
+                                      "local/solarized-theme/"))
       (require 'solarized)
       (deftheme solarized-dark "The dark variant of the Solarized colour theme")
       (deftheme solarized-light "The light variant of the Solarized colour theme"))