fix: make pretty-hydra optional

[jian-lin]

Previously, there is an error if pretty-hydra is not installed.

[jian-lin]

Steps to reproduce the error:

1. run emacs -Q --init-directory=$(mktemp -d)
2. eval this

(progn
  (require 'package)
  (push '("melpa" . "https://melpa.org/packages/") package-archives)
  (package-refresh-contents)
  (package-install 'sdml-mode))

3. go to *Compile-Log* buffer and see the error

sdml-mode-hydra.el:4:11: Error: Cannot open load file: No such file or directory, pretty-hydra