patch 8.1.2311: warning for missing function prototype

Problem:    Warning for missing function prototype.
Solution:   Add the proto. (Dominique Pelle, closes #5233)
diff --git a/src/proto/popupwin.pro b/src/proto/popupwin.pro
index 6e9b162..dafe657 100644
--- a/src/proto/popupwin.pro
+++ b/src/proto/popupwin.pro
@@ -7,6 +7,7 @@
 int popup_is_in_scrollbar(win_T *wp, int row, int col);
 void popup_handle_scrollbar_click(win_T *wp, int row, int col);
 int popup_top_extra(win_T *wp);
+int popup_left_extra(win_T *wp);
 int popup_height(win_T *wp);
 int popup_width(win_T *wp);
 int popup_extra_width(win_T *wp);
diff --git a/src/version.c b/src/version.c
index 35dc4e2..35969b5 100644
--- a/src/version.c
+++ b/src/version.c
@@ -742,6 +742,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    2311,
+/**/
     2310,
 /**/
     2309,