Add missing include to ShuffleboardEventImportance (#2310)

wpilibc/src/main/native/include/frc/shuffleboard/ShuffleboardEventImportance.h

@@ -1,5 +1,5 @@
 /*----------------------------------------------------------------------------*/
-/* Copyright (c) 2018 FIRST. All Rights Reserved.                             */
+/* Copyright (c) 2018-2020 FIRST. All Rights Reserved.                        */
 /* Open Source Software - may be modified and shared by FRC teams. The code   */
 /* must be accompanied by the FIRST BSD license file in the root directory of */
 /* the project.                                                               */
@@ -7,6 +7,8 @@
 
 #pragma once
 
+#include <wpi/StringRef.h>
+
 namespace frc {
 
 // Maintainer note: this enum is mirrored in WPILibJ and in Shuffleboard