patch 8.0.0563: crash when getting the window position in tmux

Problem:    Crash when getting the window position in tmux. (Marvin Schmidt)
Solution:   Add t_GP to the list of terminal options. (closes #1627)
diff --git a/src/option.c b/src/option.c
index d4f7275..7e1f5d1 100644
--- a/src/option.c
+++ b/src/option.c
@@ -3174,6 +3174,7 @@
     p_term("t_vi", T_VI)
     p_term("t_vs", T_VS)
     p_term("t_WP", T_CWP)
+    p_term("t_GP", T_CGP)
     p_term("t_WS", T_CWS)
     p_term("t_xn", T_XN)
     p_term("t_xs", T_XS)