Add IncludeBlocks to clang-format.

This should make "git cl format" compliant with [1].

[1] - https://google.github.io/styleguide/cppguide.html#Names_and_Order_of_Includes

Bug: None
Change-Id: Iaccae6c37965e390de8f8a3fe8e3866f51690b5a
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/204485
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Reviewed-by: Tommi <tommi@webrtc.org>
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#33151}
diff --git a/.clang-format b/.clang-format
index 11a44d5..63ebecb 100644
--- a/.clang-format
+++ b/.clang-format
@@ -19,3 +19,6 @@
 IndentWrappedFunctionNames: true
 ContinuationIndentWidth: 4
 ObjCSpaceBeforeProtocolList: true
+---
+Language: Cpp
+IncludeBlocks: Regroup