Add the bootstrapping/ dir to .gitignore

Summary: This is generated when building `ghc-cabal`.

Reviewers: hvr, bgamari, austin

Subscribers: rwbarton, thomie

Differential Revision: https://phabricator.haskell.org/D3813

.gitignore

@@ -92,6 +92,7 @@ _darcs/
 /bindistprep/
 /bindisttest/HelloWorld
 /bindisttest/
+/bootstrapping/
 /ch01.html
 /ch02.html
 /compiler/dist/