Re-add .gitignore for GN

TBR: dpranke@chromium.org
Bug: chromium:858852, v8:7900
Change-Id: I9d9bfd3434e86bd33f7a271412fc99ec727d8598
diff --git a/.gitignore b/.gitignore
index f38960e..dea1ab0 100644
--- a/.gitignore
+++ b/.gitignore
@@ -4,7 +4,10 @@
 third_party/libc++abi/trunk
 third_party/libunwind/trunk
 linux64/clang-format
+linux64/gn
 mac/clang-format
+mac/gn
 win/clang-format.exe
+win/gn.exe
 android/doclava/
 android/doclava.tar.gz