.gitignore: Add *.target.mk, generated when building as part of ChromiumOS

BUG=None
TEST=Build Chromium and ChromiumOS from source, and run "repo status",
    with and without this change.

Contributed by Josh Triplett <josh.triplett@intel.com>

git-svn-id: https://angleproject.googlecode.com/svn/trunk@1316 736b8ea6-26fd-11df-bfd4-992fa37f6226
diff --git a/.gitignore b/.gitignore
index c76963a..b42d004 100644
--- a/.gitignore
+++ b/.gitignore
@@ -6,4 +6,4 @@
 *.vcproj.*
 *.vcxproj.user
 patches-*
-
+*.target.mk