Resolve warnings by adding a return statement to a non-void function in activation_utils.h

PiperOrigin-RevId: 294507820
Change-Id: Ic226ee343d5b785289491382f793213cee8e073e

tensorflow/lite/micro/kernels/activation_utils.h

@@ -42,6 +42,8 @@ inline float ActivationValFloat(TfLiteFusedActivation act, float a) {
     case kTfLiteActSigmoid:
       return 1.0f / (1.0f + std::exp(-a));
   }
+  return 0.0f;  // To indicate an unsupported activation (i.e. when a new fused
+                // activation is added to the enum and not handled here).
 }
 
 }  // namespace micro