patch 9.0.0978: build errors without the +channel feature

Problem:    Build errors without the +channel feature. (John Marriott)
Solution:   Adjust #ifdefs.
diff --git a/src/logfile.c b/src/logfile.c
index 3ab2c0f..6c159b0 100644
--- a/src/logfile.c
+++ b/src/logfile.c
@@ -75,7 +75,7 @@
 }
 
     static void
-ch_log_lead(const char *what, channel_T *ch, ch_part_T part)
+ch_log_lead(const char *what, channel_T *ch UNUSED, ch_part_T part UNUSED)
 {
     if (log_fd == NULL)
 	return;