Add make timestamp to top level Makefile

Performed `make timestamp`.

sitemap.xml

@@ -17494,7 +17494,7 @@
 </url>
 <url>
     <loc>https://ioccc-src.github.io/temp-test-ioccc/2020/ferguson1/.gitignore</loc>
-    <lastmod>2024-03-03T06:36:22+00:00</lastmod>
+    <lastmod>2024-03-18T09:42:10+00:00</lastmod>
 </url>
 <url>
     <loc>https://ioccc-src.github.io/temp-test-ioccc/2020/ferguson1/HACKING.html</loc>
@@ -19094,11 +19094,11 @@
 </url>
 <url>
     <loc>https://ioccc-src.github.io/temp-test-ioccc/bin/gen-sitemap.sh</loc>
-    <lastmod>2024-03-18T09:22:15+00:00</lastmod>
+    <lastmod>2024-03-18T10:03:28+00:00</lastmod>
 </url>
 <url>
     <loc>https://ioccc-src.github.io/temp-test-ioccc/bin/gen-status.sh</loc>
-    <lastmod>2024-03-18T05:49:17+00:00</lastmod>
+    <lastmod>2024-03-18T10:00:50+00:00</lastmod>
 </url>
 <url>
     <loc>https://ioccc-src.github.io/temp-test-ioccc/bin/gen-top-html.sh</loc>
@@ -19370,7 +19370,7 @@
 </url>
 <url>
     <loc>https://ioccc-src.github.io/temp-test-ioccc/Makefile</loc>
-    <lastmod>2024-03-15T04:37:02+00:00</lastmod>
+    <lastmod>2024-03-18T11:06:10+00:00</lastmod>
 </url>
 <url>
     <loc>https://ioccc-src.github.io/temp-test-ioccc/news.html</loc>