Fixes pragma once warning in header files mentioned in #290

src/source_clang.cc

@@ -233,6 +233,8 @@ std::vector<std::string> Source::ClangViewParse::get_compilation_commands() {
   arguments.emplace_back("-fretain-comments-from-system-headers");
   if(file_path.extension()==".h") //TODO: temporary fix for .h-files (parse as c++)
     arguments.emplace_back("-xc++");
+  if(language && (language->get_id()=="chdr" || language->get_id()=="cpphdr"))
+    arguments.emplace_back("-Wno-pragma-once-outside-header");
   
   if(!default_build_path.empty()) {
     arguments.emplace_back("-working-directory");