Add usage message to reference_less_video_analysis.

Bug: webrtc:10616
Change-Id: Ibbaba9ec6a74466021e70d01f3a8ff4c3fd212b3
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/146205
Reviewed-by: Magnus Jedvert <magjed@webrtc.org>
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#28611}
diff --git a/rtc_tools/BUILD.gn b/rtc_tools/BUILD.gn
index f774343..a982265 100644
--- a/rtc_tools/BUILD.gn
+++ b/rtc_tools/BUILD.gn
@@ -231,6 +231,7 @@
       ":reference_less_video_analysis_lib",
       "//third_party/abseil-cpp/absl/flags:flag",
       "//third_party/abseil-cpp/absl/flags:parse",
+      "//third_party/abseil-cpp/absl/flags:usage",
     ]
   }