Use a parallel loop for IA

It sighltly helps decreasing stuttering in some area of chocolate.

src/modes/world.cpp

@@ -927,6 +927,7 @@ void World::update(float dt)
 
     PROFILER_PUSH_CPU_MARKER("World::update (AI)", 0x40, 0x7F, 0x00);
     const int kart_amount = m_karts.size();
+#pragma omp parallel for
     for (int i = 0 ; i < kart_amount; ++i)
     {
         // Update all karts that are not eliminated