[3.0.x] vlc_fixups.h: define INET6_ADDRSTRLEN for OS/2

This fixes the following compilation errors of access/rist.[ch] on OS/2:

  CC       access/librist_plugin_la-rist.lo
In file included from access/rist.c:41:
access/rist.h: In function 'populate_cname':
access/rist.h:161:22: error: 'INET6_ADDRSTRLEN' undeclared (first use in this function); did you mean 'INET_ADDRSTRLEN'?
  161 |             char str[INET6_ADDRSTRLEN];
      |                      ^~~~~~~~~~~~~~~~
      |                      INET_ADDRSTRLEN
access/rist.h:161:22: note: each undeclared identifier is reported only once for each function it appears in
access/rist.h:161:18: warning: unused variable 'str' [-Wunused-variable]
  161 |             char str[INET6_ADDRSTRLEN];
      |                  ^~~
access/rist.c: In function 'print_sockaddr_info_change':
access/rist.c:448:21: error: 'INET6_ADDRSTRLEN' undeclared (first use in this function); did you mean 'INET_ADDRSTRLEN'?
  448 |         char oldstr[INET6_ADDRSTRLEN];
      |                     ^~~~~~~~~~~~~~~~
      |                     INET_ADDRSTRLEN
access/rist.c:449:14: warning: unused variable 'newstr' [-Wunused-variable]
  449 |         char newstr[INET6_ADDRSTRLEN];
      |              ^~~~~~
access/rist.c:448:14: warning: unused variable 'oldstr' [-Wunused-variable]
  448 |         char oldstr[INET6_ADDRSTRLEN];
      |              ^~~~~~
access/rist.c: In function 'print_sockaddr_info':
access/rist.c:467:18: error: 'INET6_ADDRSTRLEN' undeclared (first use in this function); did you mean 'INET_ADDRSTRLEN'?
  467 |         char str[INET6_ADDRSTRLEN];
      |                  ^~~~~~~~~~~~~~~~
      |                  INET_ADDRSTRLEN
access/rist.c:467:14: warning: unused variable 'str' [-Wunused-variable]
  467 |         char str[INET6_ADDRSTRLEN];
      |              ^~~