Fix warnings [unused-const-variable]

../../chromecast/media/cma/backend/volume_control.cc:50:17: error:
unused variable 'kMinDbFS' [-Werror,-Wunused-const-variable]
constexpr float kMinDbFS = -120.0f;
                ^
		1 error generated.

Change-Id: I787673a625feb4d23c304404f5b62e8f663b2d6a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2131247Reviewed-by: Daniel Nicoara <dnicoara@chromium.org>
Commit-Queue: Michael Spang <spang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#755158}

chromecast/media/cma/backend/volume_control.cc

@@ -47,7 +47,9 @@ constexpr float kDefaultMediaDbFS = -25.0f;
 constexpr float kDefaultAlarmDbFS = -20.0f;
 constexpr float kDefaultCommunicationDbFS = -25.0f;
 
+#if !BUILDFLAG(SYSTEM_OWNS_VOLUME)
 constexpr float kMinDbFS = -120.0f;
+#endif
 
 constexpr char kKeyMediaDbFS[] = "dbfs.media";
 constexpr char kKeyAlarmDbFS[] = "dbfs.alarm";