patch 8.2.2577: compiler warning for type conversion

Problem:    Compiler warning for type conversion.
Solution:   Add a typecast. (Mike Williams)
diff --git a/src/drawline.c b/src/drawline.c
index 03a229e..fcb996b 100644
--- a/src/drawline.c
+++ b/src/drawline.c
@@ -1034,7 +1034,7 @@
 		    p_extra_free = alloc(MAX_MCO * fdc + 1);
 		    if (p_extra_free != NULL)
 		    {
-			n_extra = fill_foldcolumn(p_extra_free, wp,
+			n_extra = (int)fill_foldcolumn(p_extra_free, wp,
 								  FALSE, lnum);
 			p_extra_free[n_extra] = NUL;
 			p_extra = p_extra_free;