Fix mkosi on Arch Linux

/* test compression */
XZ compression finished (38280 -> 11756 bytes, 30.7%)
sh: diff: command not found
Assertion 'system(cmd) == 0' failed at src/journal/test-compress.c:198,
function test_compress_stream(). Aborting.

The journal compression test shells out to diff, so include diffutils as
a BuildPackage on Arch.

Remaining fixes in https://github.com/systemd/mkosi/pull/377

.mkosi/mkosi.arch

@@ -23,6 +23,7 @@ BuildPackages=
         cryptsetup
         curl
         dbus
+        diffutils
         docbook-xsl
         elfutils
         gcc