[CrOS MultiDevice] Add missing BleListenerFailureType::operator<<().

This function was already implemented in the .cc file, but it was not
declared in the .h file.

Bug: 824568, 752273
Change-Id: I5b8471bf5bd7bff54384696b38b10bb5523c82cf
Reviewed-on: https://chromium-review.googlesource.com/1101823
Commit-Queue: Kyle Horimoto <khorimoto@chromium.org>
Reviewed-by: Ryan Hansberry <hansberry@chromium.org>
Cr-Commit-Position: refs/heads/master@{#567481}

chromeos/services/secure_channel/ble_listener_failure_type.h

@@ -5,6 +5,8 @@
 #ifndef CHROMEOS_SERVICES_SECURE_CHANNEL_BLE_LISTENER_FAILURE_TYPE_H_
 #define CHROMEOS_SERVICES_SECURE_CHANNEL_BLE_LISTENER_FAILURE_TYPE_H_
 
+#include <ostream>
+
 namespace chromeos {
 
 namespace secure_channel {
@@ -15,6 +17,9 @@ enum class BleListenerFailureType {
   kAuthenticationError,
 };
 
+std::ostream& operator<<(std::ostream& stream,
+                         const BleListenerFailureType& failure_type);
+
 }  // namespace secure_channel
 
 }  // namespace chromeos