Fix LLDB build on old Linux kernels

Usage of aux_size is guarded against elsewhere in this file, but is missing here.

Reviewed By: wallace

Differential Revision: https://reviews.llvm.org/D110269

Original Author: calebzulawski

lldb/source/Plugins/Process/Linux/IntelPTManager.cpp

@@ -145,7 +145,11 @@ static Error CheckPsbPeriod(size_t psb_period) {
 }
 
 size_t IntelPTThreadTrace::GetTraceBufferSize() const {
+#ifndef PERF_ATTR_SIZE_VER5
+  llvm_unreachable("Intel PT Linux perf event not supported");
+#else
   return m_mmap_meta->aux_size;
+#endif
 }
 
 static Expected<uint64_t>