[buildtools] add owners

Change-Id: I2c39dbbd2a48cb2ce91333e03d876eae3342abdd
Reviewed-on: https://chromium-review.googlesource.com/c/1475145Reviewed-by: Nico Weber <thakis@chromium.org>
Reviewed-by: Thomas Anderson <thomasanderson@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Takuto Ikuta <tikuta@chromium.org>
Cr-Commit-Position: refs/heads/master@{#632835}

buildtools/OWNERS

+dpranke@chromium.org
+thakis@chromium.org
+thomasanderson@chromium.org