Add missing examples/Syntax.lpts to cabal

This is required to build from the source distribution -- this was
caught by Travis in https://travis-ci.org/Toxaris/pts/jobs/49863223. I
still have to recheck why I didn't get that locally.

pts.cabal

@@ -13,6 +13,7 @@ Extra-source-files:  examples/Arithmetics.lpts
                      examples/ChurchNumbers.lpts
                      examples/Functions.lpts
                      examples/Inference.lpts
+                     examples/Syntax.lpts
 Data-files:          emacs/pts-mode.el
 Cabal-version:       >= 1.8