Fix byte compilation warning in bindat.el

* lisp/emacs-lisp/bindat.el (bindat--unpack-item): Avoid a byte
compilation warning by using unibyte-string instead of concat +
string-make-unibyte.
1 job for master in 90 minutes and 1 second (queued for 3 seconds)
Status Job ID Name Coverage
  Test
failed #2202
test-all

01:30:01

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