vlc_fixups: define INET6_ADDRSTRLEN on OS/2

This fixes a compilation error on OS/2:

-----
  CC       stream_out/udp.lo
stream_out/udp.c: In function 'CreateSDP':
stream_out/udp.c:89:16: error: 'INET6_ADDRSTRLEN' undeclared (first use in this function); did you mean 'INET_ADDRSTRLEN'?
   89 |     char dhost[INET6_ADDRSTRLEN];
      |                ^~~~~~~~~~~~~~~~
      |                INET_ADDRSTRLEN
stream_out/udp.c:89:16: note: each undeclared identifier is reported only once for each function it appears in
stream_out/udp.c:89:10: warning: unused variable 'dhost' [-Wunused-variable]
   89 |     char dhost[INET6_ADDRSTRLEN];
      |          ^~~~~
-----

include/vlc_fixups.h

@@ -637,6 +637,8 @@ struct sockaddr_in6
 
 # define IN6_IS_ADDR_MULTICAST(a)   (((__const uint8_t *) (a))[0] == 0xff)
 
+# define INET6_ADDRSTRLEN   46
+
 static const struct in6_addr in6addr_any =
     { { 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0 } };