hal: Fix build error for battery listner module

- Build error is observed with -Wimplicit-fallthrough
  compile time flag enabled

- Add explicit fallthrough in switch statement

Change-Id: Icea383983e689fcbf5579a5d8de1cc0fa65f610f
diff --git a/hal/audio_extn/battery_listener.cpp b/hal/audio_extn/battery_listener.cpp
index 582e9e6..5f19443 100644
--- a/hal/audio_extn/battery_listener.cpp
+++ b/hal/audio_extn/battery_listener.cpp
@@ -140,6 +140,7 @@
                         if (mStatusnot_ncharging()) // i.e event changed
                             break;
                     }
+                    [[fallthrough]];
                     default:
                         bool c = statusToBool(local_status);
                         ALOGI("healthInfo cb thread: cb %s", c ? "CHARGING" : "NOT CHARGING");