Ignore unknown-pragmas warning of clang

This was previously ignored through Chromium until
https://chromium-review.googlesource.com/984113

Bug: None
Change-Id: I4bc402e32d888fc2536ad69c832130c4086b5adb
Reviewed-on: https://webrtc-review.googlesource.com/66040
Reviewed-by: Patrik Höglund <phoglund@webrtc.org>

BUILD.gn

diff --git a/BUILD.gn b/BUILD.gn
index 5f620ca..291a208 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -30,6 +30,7 @@
         "-Wno-reorder",
         "-Wno-string-conversion",
         "-Wno-tautological-constant-out-of-range-compare",
+        "-Wno-unknown-pragmas",
         "-Wno-unused-private-field",
         "-Wno-writable-strings",
       ]