patch 8.1.0770: inconsistent use of ELAPSED_FUNC

Problem:    Inconsistent use of ELAPSED_FUNC.
Solution:   Consistently use ELAPSED_FUNC.  Also turn ELAPSED_TYPE into a
            typedef. (Ozaki Kiichi, closes #3815)
diff --git a/src/channel.c b/src/channel.c
index e08081d..8f0577f 100644
--- a/src/channel.c
+++ b/src/channel.c
@@ -4290,7 +4290,7 @@
     int		r;
     ch_part_T	part = PART_SOCK;
 #ifdef ELAPSED_FUNC
-    ELAPSED_TYPE  start_tv;
+    elapsed_T	start_tv;
 
     ELAPSED_INIT(start_tv);
 #endif