.gitignore: Keep Sal source code in repository

Explanation from Roger Dannenberg: The original Nyquist language was an
extended XLISP. Now, there are two "syntaxes": Lisp and Sal. Sal is a
procedural infix language. A small compiler (written in Lisp) translates
Sal into Lisp and then the translated code runs directly on the Lisp
interpreter. You can write plug-ins in either Lisp or Sal syntax, and
there could be library code to support plug-ins and provide other
functions in the future. Sal is definitely source code and likely to be
compact. I would advise any new user to use Sal instead of Lisp, so I
expect more .sal sources in the future.

Thus keep the Sal source code in the repository and do not ignore them.

.gitignore

@@ -108,8 +108,6 @@ mac/tests/
 *.tlog
 *.ipch
 *.opensdf
-# unsure about the .sal files.  Disable for now.
-*.sal
 *.vcxproj.user
 
 # Precompiled Headers