GPU: Fix compilation without O2 headers

Author: davidrohr

GPU/GPUTracking/Base/GPUO2FakeClasses.h

@@ -107,6 +107,8 @@ class GPUTrackingRefitProcessor
 };
 struct GPUTPCCFChainContext {
 };
+struct TPCPadGainCalib {
+};
 #ifndef __OPENCL__
 struct GPUParam;
 class GPUTPCClusterStatistics

GPU/GPUTracking/Global/GPUChainTracking.cxx

@@ -2704,7 +2704,9 @@ void GPUChainTracking::ClearErrorCodes()
 
 void GPUChainTracking::SetDefaultO2PropagatorForGPU()
 {
+#ifdef HAVE_O2HEADERS
   o2::base::Propagator* prop = param().GetDefaultO2Propagator(true);
   prop->setMatLUT(processors()->calibObjects.matLUT);
   SetO2Propagator(prop);
+#endif
 }