patch 8.2.3958: build failure compiling xxd with "-std=c2x"

Problem:    Build failure compiling xxd with "-std=c2x".
Solution:   define _XOPEN_SOURCE. (Yegappan Lakshmanan, closes #9444)
diff --git a/src/version.c b/src/version.c
index 0431474..80f5048 100644
--- a/src/version.c
+++ b/src/version.c
@@ -750,6 +750,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    3958,
+/**/
     3957,
 /**/
     3956,