Require f in lsp-treemacs-themes (#126)

It was already required in lsp-treemacs, but lsp-treemacs-themes
does not require that either.

Author: tarsius

lsp-treemacs-themes.el

@@ -23,6 +23,7 @@
 ;;
 ;;; Code:
 
+(require 'f)
 (require 'treemacs)
 (require 'treemacs-themes)