Add buildtools/ to .gitignore

BUG=none
R=machenbach@chromium.org
LOG=n

Review URL: https://codereview.chromium.org/312643004

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21638 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

.gitignore

@@ -39,6 +39,7 @@ shell_g
 /build/gyp
 /build/ipch/
 /build/Release
+/buildtools
 /hydrogen.cfg
 /obj
 /out