Changed to C linkage in netdb.h for fixing bug when using mixed C/C++ code (IDFGH-4027)

[mathiasbredholt]

I got an error when trying to compile a project with mixed C/C++ code. This fixes the linkage issues. The error message is:

error: conflicting declaration of 'int getnameinfo(const sockaddr*, socklen_t, char*, socklen_t, char*, socklen_t, int)' with 'C' linkage
 int getnameinfo(const struct sockaddr *addr, socklen_t addrlen,
     ^~~~~~~~~~~

[CLAassistant]

All committers have signed the CLA.

[Alvin1Zhang]

Thanks for your contribution.

[0xjakob]

@mathiasbredholt The change is a good catch! Thanks for providing this to us. I just have a tiny comment regarding the order of #ifdef and C++ guards.

[Alvin1Zhang]

Thanks for your contribution again, changes have been merged with 8a10292