Fix no_global_constructors in audio_processing/agc2/rnn_vad.

This is a rework of [1] following kwiberg@'s advice.

[1] - https://webrtc-review.googlesource.com/c/src/+/98583

Bug: webrtc:9693
Change-Id: I8d4fac8d7593c28d4ad2a973637f965f2cd51e99
Reviewed-on: https://webrtc-review.googlesource.com/98881
Reviewed-by: Alessio Bazzica <alessiob@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#24667}

modules/audio_processing/agc2/rnn_vad/BUILD.gn

@@ -9,7 +9,6 @@
 import("../../../../webrtc.gni")
 
 rtc_source_set("rnn_vad") {
-  configs += [ "../../../..:no_global_constructors" ]
   sources = [
     "common.h",
     "features_extraction.cc",

modules/audio_processing/agc2/rnn_vad/spectral_features_internal.cc

@@ -20,7 +20,10 @@ namespace rnn_vad {
 namespace {
 
 // DCT scaling factor.
-const float kDctScalingFactor = std::sqrt(2.f / kNumBands);
+static_assert(
+    kNumBands == 22,
+    "kNumBands changed! Please update the value of kDctScalingFactor");
+constexpr float kDctScalingFactor = 0.301511345f;  // sqrt(2 / kNumBands)
 
 }  // namespace