patch 9.1.1284: not possible to configure pum truncation char

Problem:  not possible to configure the completion menu truncation
          character
Solution: add the "trunc" suboption to the 'fillchars' setting to
          configure the truncation indicator (glepnir).

closes: #17006

Co-authored-by: zeertzjq <zeertzjq@outlook.com>
Signed-off-by: glepnir <glephunter@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/structs.h b/src/structs.h
index 78f0a70..9b44598 100644
--- a/src/structs.h
+++ b/src/structs.h
@@ -3850,6 +3850,7 @@
     int	diff;
     int	eob;
     int	lastline;
+    int trunc;
 } fill_chars_T;
 
 /*