ensure built en-manpages are ignored by git

Seems like the interaction between wildcards and exemptions has changed
slightly, so be positively more explicit about our ignoring of en.

Git-Dch: Ignore

.gitignore

@@ -15,6 +15,7 @@
 /doc/*/
 !/doc/examples/
 !/doc/po/
+/doc/en/*
 !/doc/en/makefile
 
 # package building artifacts