.gitignore: add prism(-sources).jar and associated temporary file

Blacklist the following files from version control:

* prism/lib/prism.jar: this is generated by the "binary" target in
  prism/Makefile.
* prism/lib/prism-sources.jar: this is generated by the "source-jar"
  target in prism/Makefile.
* prism/prism-sources.txt: this is a temporary build file generated by
  the "source-jar" target in prism/Makefile and will be deleted if it
  succeeds, but will be left behind if jar(1) exits unsuccessfully.

.gitignore

@@ -19,8 +19,13 @@ prism/bin/
 prism/classes/
 prism/obj/
 
+# temporary build files:
+prism/prism-sources.txt
+
 # generated JAR files:
 prism/lib/lpsolve55j.jar
+prism/lib/prism.jar
+prism/lib/prism-sources.jar
 
 # generated CUDD headers:
 cudd/include/*.h