patch 8.2.0338: build failure without the channel feature

Problem:    Build failure without the channel feature.
Solution:   Add #ifdef
diff --git a/src/vim9compile.c b/src/vim9compile.c
index a4525ca..2e0c2ff 100644
--- a/src/vim9compile.c
+++ b/src/vim9compile.c
@@ -5227,11 +5227,15 @@
 	    break;
 
 	case ISN_PUSHJOB:
+#ifdef FEAT_JOB_CHANNEL
 	    job_unref(isn->isn_arg.job);
+#endif
 	    break;
 
 	case ISN_PUSHCHANNEL:
+#ifdef FEAT_JOB_CHANNEL
 	    channel_unref(isn->isn_arg.channel);
+#endif
 	    break;
 
 	case ISN_UCALL: