Removed warnings of unused variable in btThreads header

src/LinearMath/btThreads.h

@@ -72,6 +72,8 @@ SIMD_FORCE_INLINE void btMutexLock( btSpinMutex* mutex )
 {
 #if BT_THREADSAFE
     mutex->lock();
+#else
+    (void)mutex;
 #endif // #if BT_THREADSAFE
 }
 
@@ -79,6 +81,8 @@ SIMD_FORCE_INLINE void btMutexUnlock( btSpinMutex* mutex )
 {
 #if BT_THREADSAFE
     mutex->unlock();
+#else
+    (void)mutex;
 #endif // #if BT_THREADSAFE
 }
 
@@ -87,6 +91,7 @@ SIMD_FORCE_INLINE bool btMutexTryLock( btSpinMutex* mutex )
 #if BT_THREADSAFE
     return mutex->tryLock();
 #else
+    (void)mutex;
     return true;
 #endif // #if BT_THREADSAFE
 }