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/DigitalPWMDataInternal.h

@@ -10,6 +10,8 @@
 #include <atomic>
 #include <memory>
 
+#include <support/mutex.h>
+
 #include "MockData/DigitalPWMData.h"
 #include "MockData/NotifyListenerVector.h"
 
@@ -40,7 +42,7 @@ class DigitalPWMData {
   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<double> m_dutyCycle{false};