(gui-get-selection): Avoid using `string-to-multibyte'

* lisp/select.el (gui-get-selection): Avoid using
`string-to-multibyte' -- instead encode as `eight-bit' if the
string is unibyte.
1 job for master in 90 minutes and 4 seconds (queued for 5 seconds)
Status Job ID Name Coverage
  Test
failed #1792
test-all

01:30:04

 
Name Stage Failure
failed
test-all Test The script exceeded the maximum execution time set for the job
  GEN      lisp/emacs-lisp/cl-macs-tests.log
ELC lisp/emacs-lisp/cl-preloaded-tests.elc
GEN lisp/emacs-lisp/cl-preloaded-tests.log
ELC lisp/emacs-lisp/cl-print-tests.elc
GEN lisp/emacs-lisp/cl-print-tests.log
ELC lisp/emacs-lisp/cl-seq-tests.elc
GEN lisp/emacs-lisp/cl-seq-tests.log
Pulling docker image gitlab/gitlab-runner-helper:x86_64-f100a208 ...
ERROR: Job failed: execution took longer than 1h30m0s seconds