mkosi: update Debian config for mkosi (#5997)

* mkosi.debian: fix libidn2 package name
* mkosi.debian: drop duplicate diffutils package
* mkosi.debian: add missing g++ package

.mkosi/mkosi.debian

@@ -32,9 +32,9 @@ RootSize=2G
 [Packages]
 BuildPackages=
         acl
-        diffutils
         docbook-xml
         docbook-xsl
+        g++
         gcc
         git
         gnu-efi
@@ -53,7 +53,7 @@ BuildPackages=
         libfdisk-dev
         libgcrypt20-dev
         libgnutls28-dev
-        libidn2-dev
+        libidn2-0-dev
         libkmod-dev
         liblzma-dev
         liblz4-dev