fix. backport compilation with llvm9 and earlier

llvm::SectionRef changed the `getName` interface since LLVM10, in
particular, in commit llvm/llvm-project@a11d302fa0.

In llvm-9 and early version, `getName` required a StringRef reference as
a parameter, and returned a std::error_code.

src/cc/bpf_module.cc

@@ -122,10 +122,17 @@ class MyMemoryManager : public SectionMemoryManager {
  if (!section)
  continue;
 
+#if LLVM_MAJOR_VERSION >= 10
  auto sec_name = section.get()->getName();
  if (!sec_name)
  continue;
+#else
+ llvm::StringRef sec_name_obj;
+ if (!section.get()->getName(sec_name_obj))
+ continue;
 
+ auto sec_name = &sec_name_obj;
+#endif
  info->section_ = sec_name->str();
  info->size_ = ss.second;
  }