Add the lex source to toybox release

This was an oversight
diff --git a/toybox/files.txt b/toybox/files.txt
index 6fe999a..c82df05 100644
--- a/toybox/files.txt
+++ b/toybox/files.txt
@@ -12,6 +12,7 @@
 src/bc/bc.c
 src/bc/data.c
 src/bc/io.c
+src/bc/lex.c
 src/bc/num.c
 src/bc/parse.c
 src/bc/program.c