patch 8.1.0762: compiler warning

Problem:    Compiler warning.
Solution:   Add type cast. (Mike Williams)
diff --git a/src/channel.c b/src/channel.c
index 0cec71b..e08081d 100644
--- a/src/channel.c
+++ b/src/channel.c
@@ -4064,7 +4064,7 @@
     else
     {
 	text = tv_get_string_buf(&argvars[1], buf);
-	len = STRLEN(text);
+	len = (int)STRLEN(text);
     }
     channel = send_common(argvars, text, len, 0, eval, &opt,
 			      eval ? "ch_evalraw" : "ch_sendraw", &part_read);