repohooks: allow tabs in .mk files

BUG=None
TEST=Adhoc
'repo upload' with a CL with tabs in a .mk file.

Change-Id: I845e21a893611ff0ccb24f4ea7db700ad5cb90bf
Signed-off-by: Elly Jones <ellyjones@chromium.org>
Reviewed-on: https://gerrit.chromium.org/gerrit/11718
Reviewed-by: Will Drewry <wad@chromium.org>
diff --git a/pre-upload.py b/pre-upload.py
index 77ea6fe..2548572 100644
--- a/pre-upload.py
+++ b/pre-upload.py
@@ -228,7 +228,8 @@
       r"/src/third_party/u-boot/",
       r".*\.ebuild$",
       r".*\.eclass$",
-      r".*/[M|m]akefile$"
+      r".*/[M|m]akefile$",
+      r".*\.mk$"
   ]
 
   errors = []