Add stdspecialchars.inc to Makefile.am

This might fix 'make distcheck' (at least for me), which was failing
with a mysterious error message:

  cannot remove ‘../../po/lyx.pot’: Permission denied

For discussion, see:

  https://www.mail-archive.com/search?l=mid&q=20151125075443.GA7491%40cotopaxi.hsd1.dc.comcast.net

I write "might" because I have no idea why in theory it would. But
for whatever reason it worked when I tested.

lib/Makefile.am

@@ -2124,6 +2124,7 @@ dist_layouts_DATA =\
 	layouts/stdlyxlist.inc \
 	layouts/stdrefprefix.inc \
 	layouts/stdsections.inc \
+	layouts/stdspecialchars.inc \
 	layouts/stdstarsections.inc \
 	layouts/stdstruct.inc \
 	layouts/stdtitle.inc \