patch 8.0.0712: the terminal implementation is incomplete

Problem:    The terminal implementation is incomplete.
Solution:   Add the 'termkey' option.
diff --git a/src/option.h b/src/option.h
index 69fc348..c74a89b 100644
--- a/src/option.h
+++ b/src/option.h
@@ -1131,6 +1131,7 @@
     , WV_COLE
 #endif
 #ifdef FEAT_TERMINAL
+    , WV_TK
     , WV_TMS
 #endif
 #ifdef FEAT_CURSORBIND