Add usage message to psnr_ssim_analyzer.

Bug: webrtc:10616
Change-Id: I157fbf63b3794a1f655901ace7355edb1240cc72
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/146203
Reviewed-by: Magnus Jedvert <magjed@webrtc.org>
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#28610}
diff --git a/rtc_tools/BUILD.gn b/rtc_tools/BUILD.gn
index 28a0547..f774343 100644
--- a/rtc_tools/BUILD.gn
+++ b/rtc_tools/BUILD.gn
@@ -202,6 +202,7 @@
       "../api/video:video_rtp_headers",
       "//third_party/abseil-cpp/absl/flags:flag",
       "//third_party/abseil-cpp/absl/flags:parse",
+      "//third_party/abseil-cpp/absl/flags:usage",
     ]
   }