test-gendoc: require gettext

If the gettext utilities aren't installed, there is no way to make the
translations.  This causes the gettext client to fall back to the untranslated
message, which triggers "** NOTHING TRANSLATED **" errors and a test failure.

tests/test-gendoc.t

@@ -1,4 +1,5 b''
 #require docutils
+#require gettext
 
 Test document extraction