Use wpi::mutex instead of std::mutex. (#730)

This uses a priority-aware mutex on Linux platforms.

Fixes #729.

hal/src/main/native/sim/MockData/EncoderDataInternal.h

@@ -11,6 +11,8 @@
 #include <limits>
 #include <memory>
 
+#include <support/mutex.h>
+
 #include "MockData/EncoderData.h"
 #include "MockData/NotifyListenerVector.h"
 
@@ -76,7 +78,7 @@ class EncoderData {
   virtual void ResetData();
 
  private:
-  std::mutex m_registerMutex;
+  wpi::mutex m_registerMutex;
   std::atomic<HAL_Bool> m_initialized{false};
   std::shared_ptr<NotifyListenerVector> m_initializedCallbacks = nullptr;
   std::atomic<int32_t> m_count{0};