[master] add missing dist file

detected by distcheck failure:
http://git.bind10.isc.org/~tester/builder/BIND10-distcheck/20120831093000-FreeBSD8-i386/logs/distcheck.out

discussed on jabber

src/lib/datasrc/tests/Makefile.am

@@ -105,6 +105,7 @@ EXTRA_DIST += testdata/contexttest.zone
 EXTRA_DIST += testdata/diffs.sqlite3
 EXTRA_DIST += testdata/example2.com
 EXTRA_DIST += testdata/example2.com.sqlite3
+EXTRA_DIST += testdata/example.com.flattened
 EXTRA_DIST += testdata/example.com.signed
 EXTRA_DIST += testdata/example.org
 EXTRA_DIST += testdata/example.org.nsec3-signed