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 <[email protected]>
Reviewed-by: Tommi <[email protected]>
Commit-Queue: Mirko Bonadei <[email protected]>
Cr-Commit-Position: refs/heads/master@{#33151}

Author: MirkoBonadei

.clang-format

@@ -19,3 +19,6 @@ BreakBeforeTernaryOperators: false
 IndentWrappedFunctionNames: true
 ContinuationIndentWidth: 4
 ObjCSpaceBeforeProtocolList: true
+---
+Language: Cpp
+IncludeBlocks: Regroup