patch 9.0.0979: ch_log() text can be hard to find in the log file

Problem:    ch_log() text can be hard to find in the log file.
Solution:   Prepend "ch_log()" to the text.
diff --git a/src/logfile.c b/src/logfile.c
index 6c159b0..673468e 100644
--- a/src/logfile.c
+++ b/src/logfile.c
@@ -177,7 +177,9 @@
 	channel = get_channel_arg(&argvars[1], FALSE, FALSE, 0);
 #endif
 
-    ch_log(channel, "%s", msg);
+    // Prepend "ch_log()" to make it easier to find these entries in the
+    // logfile.
+    ch_log(channel, "ch_log(): %s", msg);
 }
 
 /*