Add missing aux-file in Makefile.

Fixes installation of ungoogled-chromium on foreign distros.

This is a followup to commit f1e9de4d3a.

* Makefile.am (AUX_FILES): Add missing
gnu/packages/aux-files/chromium/master-preferences.json entry.

Signed-off-by: Leo Famulari <leo@famulari.name>

Makefile.am

@@ -280,6 +280,7 @@ dist_noinst_DATA = guix/tests.scm guix/tests/http.scm
 
 # Auxiliary files for packages.
 AUX_FILES =						\
+  gnu/packages/aux-files/chromium/master-preferences.json		\
   gnu/packages/aux-files/emacs/guix-emacs.el		\
   gnu/packages/aux-files/linux-libre/4.20-arm.conf	\
   gnu/packages/aux-files/linux-libre/4.20-arm64.conf	\