Add usage message to rgba_to_i420_converter.

Bug: webrtc:10616
Change-Id: Id8268bada4ec42130d9b7b105da73218dfa43519
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/146209
Reviewed-by: Magnus Jedvert <magjed@webrtc.org>
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#28614}
diff --git a/rtc_tools/BUILD.gn b/rtc_tools/BUILD.gn
index a982265..18f498f 100644
--- a/rtc_tools/BUILD.gn
+++ b/rtc_tools/BUILD.gn
@@ -248,6 +248,7 @@
       "../common_video",
       "//third_party/abseil-cpp/absl/flags:flag",
       "//third_party/abseil-cpp/absl/flags:parse",
+      "//third_party/abseil-cpp/absl/flags:usage",
       "//third_party/libyuv",
     ]
   }