cc: Fix compile warning (iovisor#2346)

Add corresponding ifdef macro to get ride of the follow warning.

/home/lecopzer/workspace/mybcc/src/cc/bcc_debug.cc: In member function ‘void ebpf::SourceDebugger::dump()’:
/home/lecopzer/workspace/mybcc/src/cc/bcc_debug.cc:186:16: warning: unused variable ‘SectionID’ [-Wunused-variable]
       unsigned SectionID = get<2>(section.second);
                ^~~~~~~~~

src/cc/bcc_debug.cc

@@ -183,7 +183,9 @@ void SourceDebugger::dump() {
  uint64_t Size;
  uint8_t *FuncStart = get<0>(section.second);
  uint64_t FuncSize = get<1>(section.second);
+#if LLVM_MAJOR_VERSION >= 9
  unsigned SectionID = get<2>(section.second);
+#endif
  ArrayRef<uint8_t> Data(FuncStart, FuncSize);
  uint32_t CurrentSrcLine = 0;
  string func_name = section.first.substr(fn_prefix_.size());