document that media_codecs_sw.xml is part of mainline

Bug: 247992660
Test: repo upload
Change-Id: I49c3636d3b01acb5f3d0ba7c4a02a565486ded3a
diff --git a/MainlineFiles.cfg b/MainlineFiles.cfg
index 4e1d3a2..bf39c1a 100644
--- a/MainlineFiles.cfg
+++ b/MainlineFiles.cfg
@@ -30,6 +30,7 @@
 media/codec2/hidl/
 media/codec2/sfplugin/utils/
 media/codec2/vndk/
+media/libstagefright/data/media_codecs_sw.xml
 media/module/
 services/mediacodec/