luacheck: added 'coroutine._yield' to read-only globals.

This fixes builds with Luacheck 0.19.0+.

Signed-off-by: Yichun Zhang (agentzh) <agentzh@gmail.com>

Author: thibaultcha

.luacheckrc

@@ -1,2 +1,5 @@
 std = 'ngx_lua'
 unused_args = false
+read_globals = {
+    "coroutine._yield"
+}