GN: Add audio_conference_mixer_unittests to modules_unittests.

webrtc/modules/audio_conference_mixer/BUILD.gn

Index: webrtc/modules/audio_conference_mixer/BUILD.gn
diff --git a/webrtc/modules/audio_conference_mixer/BUILD.gn b/webrtc/modules/audio_conference_mixer/BUILD.gn
index 3c6f3d34d7b68a497cb13bbe5227feee48d289c9..b4705d53290ba70c3bd7e797a85cdc770b359837 100644
--- a/webrtc/modules/audio_conference_mixer/BUILD.gn
+++ b/webrtc/modules/audio_conference_mixer/BUILD.gn
@@ -58,6 +58,12 @@ if (rtc_include_tests) {
       "../../test:test_support",
       "//testing/gmock",
     ]
+    if (is_win) {
+      cflags = [
+        # TODO(kjellander): bugs.webrtc.org/261: Fix this warning.
+        "/wd4373",  # virtual function override.
+      ]
+    }
     if (!build_with_chromium && is_clang) {
       # Suppress warnings from the Chromium Clang plugin (bugs.webrtc.org/163).
       suppressed_configs += [ "//build/config/clang:find_bad_constructs" ]