Fix compilation warning about gv-setter in cl-generic

* lisp/emacs-lisp/cl-generic.el (gv-setter): Declare to avoid a
compilation warning.
1 job for master in 76 minutes and 15 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test
failed #2229
test-all

01:16:15

 
Name Stage Failure
failed
test-all Test
Makefile:318: recipe for target 'check-doit' failed
make[2]: *** [check-doit] Error 1
make[2]: Leaving directory '/builds/emacs/emacs/test'
Makefile:293: recipe for target 'check-expensive' failed
make[1]: Leaving directory '/builds/emacs/emacs/test'
make[1]: *** [check-expensive] Error 2
Makefile:957: recipe for target 'check-expensive' failed
make: *** [check-expensive] Error 2
ERROR: Job failed: exit code 1