2.99.1: man pages - incorrect install path

Looks like currently all *.1 man pages are installed in mandir instead in mandir/man1. Possible fix:

--- a/docs/manpages/meson.build~        2020-05-03 15:03:38.000000000 +0100
+++ b/docs/manpages/meson.build 2020-05-04 07:24:42.978686403 +0100
@@ -32,6 +32,6 @@
     input: xml,
     output: manpage,
     install: true,
-    install_dir: get_option('mandir'),
+    install_dir: join_paths(get_option('mandir'), 'man1'),
   )
 endforeach