[3.0] vlc_fixups.h: include sys/socket.h on OS/2

OS/2 LIBCn(a fork of kLIBC) has socklen_t in sys/socket.h.

This fixes the following compilation error:

  CC       freeaddrinfo.lo
In file included from ../config.h:981,
                 from freeaddrinfo.c:27:
../include/vlc_fixups.h:599:47: error: unknown type name 'socklen_t'; did you mean 'locale_t'?
  599 | int  getnameinfo  (const struct sockaddr *sa, socklen_t salen,
      |                                               ^~~~~~~~~
      |                                               locale_t