Fix calculation of target bitrate of VP9 spatial layer.

This fixes misprint in the code which calculates target bitrate of a
VP9 spatial layer where "-" was used instead of "+".

Bug: none
Change-Id: I17d76a84d00e453c055c068968d7b276e9c23f51
Reviewed-on: https://webrtc-review.googlesource.com/71663
Commit-Queue: Sergey Silkin <ssilkin@webrtc.org>
Reviewed-by: Rasmus Brandt <brandtr@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#22974}
diff --git a/modules/video_coding/BUILD.gn b/modules/video_coding/BUILD.gn
index b8022e7..3a37bcb 100644
--- a/modules/video_coding/BUILD.gn
+++ b/modules/video_coding/BUILD.gn
@@ -745,6 +745,7 @@
       "codecs/vp8/default_temporal_layers_unittest.cc",
       "codecs/vp8/screenshare_layers_unittest.cc",
       "codecs/vp8/simulcast_unittest.cc",
+      "codecs/vp9/svc_config_unittest.cc",
       "codecs/vp9/svc_rate_allocator_unittest.cc",
       "decoding_state_unittest.cc",
       "fec_controller_unittest.cc",