patch 9.0.0983: stray characters displayed when starting the GUI

Problem:    Stray characters displayed when starting the GUI.
Solution:   Add t_RK to the list of terminal options.
diff --git a/src/optiondefs.h b/src/optiondefs.h
index e7eb357..449e7ef 100644
--- a/src/optiondefs.h
+++ b/src/optiondefs.h
@@ -2897,6 +2897,7 @@
     p_term("t_RC", T_CRC)
     p_term("t_RI", T_CRI)
     p_term("t_Ri", T_SRI)
+    p_term("t_RK", T_CRK)
     p_term("t_RS", T_CRS)
     p_term("t_RT", T_CRT)
     p_term("t_RV", T_CRV)