meson: Avoid duplicates in generated config-poison.h again

Commit eed56e9a89 "configure, meson: move config-poison.h to meson"
lost a "| sort -u".  Restore it.  config-poison shrinks from ~4500 to
~700 lines when all targets are enabled.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Tested-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20230206102037.3621709-1-armbru@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

scripts/make-config-poison.sh

@@ -13,4 +13,4 @@ exec sed -n \
   -e    's///' \
   -e    's/ .*//' \
   -e    's/^/#pragma GCC poison /p' \
-  -e '}' "$@"
+  -e '}' "$@" | sort -u