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

This uses a priority-aware mutex on Linux platforms.

Fixes #729.

wpilibc/src/main/native/include/PIDController.h

@@ -9,10 +9,11 @@
 
 #include <atomic>
 #include <memory>
-#include <mutex>
 #include <queue>
 #include <string>
 
+#include <support/mutex.h>
+
 #include "Base.h"
 #include "Controller.h"
 #include "LiveWindow/LiveWindow.h"
@@ -147,7 +148,7 @@ class PIDController : public LiveWindowSendable, public PIDInterface {
   std::queue<double> m_buf;
   double m_bufTotal = 0;
 
-  mutable std::mutex m_mutex;
+  mutable wpi::mutex m_mutex;
 
   std::unique_ptr<Notifier> m_controlLoop;
   Timer m_setpointTimer;