List of all C++ incompatibility caused by extern C guards in 0.19.0

[marty1885]

Hi,

I found more C++ incompatibility while adding GNS support to my C++ program. I 
wiped a quick awk line to scan (by counting if there's an even count of guard 
attempts). It shows two more files have the same issue

```
❯ cd /usr/include/gnunet
❯ for f in `find . -type f`; do awk '/\/\* keep Emacse/{count += 1} END{ if 
(count % 2 != 0) print FILENAME}' $f; done
./gnunet_gnsrecord_json_lib.h
./gnu_name_system_record_flags.h
./gnunet_container_lib.h
```
gnunet_container_lib.h was reported in my previous post. I believe this is the 
entire list of files with bad extern C guards. Can someone with git access 
patch it?


Thanks,
Martin