[chrony] fix typo in GNUmakefile (avoids issues in macOS)

1 job for master in 89 minutes and 49 seconds (queued for 2 seconds)
Status Name Job ID Coverage
  Test
failed build #423034
docker

01:29:49

 
Name Stage Failure
failed
build Test
 If = 2644
Loop = 479
Goto = 540
-Assignment = 5172
+Assignment = 5162
Exit point = 246
Function = 246
Function call = 3745
ERROR: Job failed: exit code 1