Add .clang-format to hidl-gen.

This is the same clang format used in hardware/interfaces.

Test: attempt upload for change, see warning
Change-Id: I01d3715437bd243b656611e0d62acb77d5fc6b2d
diff --git a/PREUPLOAD.cfg b/PREUPLOAD.cfg
new file mode 100644
index 0000000..213c93a
--- /dev/null
+++ b/PREUPLOAD.cfg
@@ -0,0 +1,5 @@
+[Options]
+ignore_merged_commits = true
+
+[Builtin Hooks]
+clang_format = true