Recognize `.nix' files as text files.

--- idutils-4.5/libidu/id-lang.map	2009-05-17 16:38:42.000000000 +0200
+++ idutils-4.5/libidu/id-lang.map	2010-07-04 15:52:48.000000000 +0200
@@ -88,6 +88,7 @@ GNUmakefile		make
 *.doc			text
 *.txt			text
 *.org			text
+*.nix			text
 
 *.m4			m4
 *.ac			m4