add tarballs to .gitignore

Summary:
@public
release tarballs generated by our scripts should be ignored.

Test Plan:
put all the tarballs in infer/ (sources, osx, linux), git status doesn't show
them.

.gitignore

@@ -44,6 +44,9 @@ infer/tests/.idea/copyright/profiles_settings.xml
 infer-out/
 *.o.astlog
 *.o.sh
+infer-sources.tar.gz
+infer-osx-*.tar.xz
+infer-linux64-*.tar.xz
 
 # Directories generated by buck
 buck-out/