Add empty placeholder target for rms_level.*

A follow-up CL will move the rms_level.* files into the new target.

Bug: webrtc:11226
Change-Id: I59579b026346f627c0a2739d25f90c12bffbf248
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/171102
Commit-Queue: Sam Zackrisson <saza@webrtc.org>
Reviewed-by: Per Åhgren <peah@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#30840}

modules/audio_processing/BUILD.gn

@@ -157,6 +157,7 @@ rtc_library("audio_processing") {
     ":audio_processing_statistics",
     ":config",
     ":high_pass_filter",
+    ":rms_level",
     ":voice_detection",
     "../../api:array_view",
     "../../api:function_view",
@@ -213,6 +214,14 @@ rtc_library("voice_detection") {
   ]
 }
 
+rtc_source_set("rms_level") {
+  visibility = [ "*" ]
+
+  # TODO(webrtc:11226): Move rms_level.* to here.
+  sources = []
+  deps = []
+}
+
 rtc_library("audio_processing_statistics") {
   visibility = [ "*" ]
   sources = [