patch 8.2.4569: Coverity warning for not using a return value

Problem:    Coverity warning for not using a return value.
Solution:   Add "(void)".
diff --git a/src/popupwin.c b/src/popupwin.c
index 9dee757..0aca7d3 100644
--- a/src/popupwin.c
+++ b/src/popupwin.c
@@ -3738,7 +3738,7 @@
 				{
 				    // compute the position in the buffer line
 				    // from the position in the window
-				    mouse_comp_pos(wp, &line_cp, &col_cp,
+				    (void)mouse_comp_pos(wp, &line_cp, &col_cp,
 							  &lnum, plines_cache);
 				    redrawWinline(wp, lnum);
 				}