patch 8.1.1416: popup_getposition() not implemented yet

Problem:    Popup_getposition() not implemented yet.
Solution:   Implement it. (Yasuhiro Matsumoto, closes #4449)
diff --git a/src/evalfunc.c b/src/evalfunc.c
index ccced48..1a0e113 100644
--- a/src/evalfunc.c
+++ b/src/evalfunc.c
@@ -810,6 +810,7 @@
 #ifdef FEAT_TEXT_PROP
     {"popup_close",	1, 1, f_popup_close},
     {"popup_create",	2, 2, f_popup_create},
+    {"popup_getposition", 1, 1, f_popup_getposition},
     {"popup_hide",	1, 1, f_popup_hide},
     {"popup_move",	2, 2, f_popup_move},
     {"popup_show",	1, 1, f_popup_show},