flymake: fix typo in variable binding (bug#38752)

This mistake was found by an experimental elisp optimiser.

* lisp/progmodes/flymake-proc.el (flymake-proc-stop-all-syntax-checks):
Add missing brackets.
1 job for emacs-27 in 55 minutes and 26 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Test
passed #4728
test-all

00:55:26