Always include G_SOCKET_FAMILY_UNIX value in GSocketFamily

This is needed because glib-mkenums doesn't handle #ifdef values in
enums, and so it needs to have all values always defined in the enum.
When not available, define the missing values to a negative value.

gio/gioenums.h

@@ -687,9 +687,7 @@ typedef enum /*< flags >*/ {
  */
 typedef enum {
   G_SOCKET_FAMILY_INVALID,
-#ifdef GLIB_SYSDEF_AF_UNIX
   G_SOCKET_FAMILY_UNIX = GLIB_SYSDEF_AF_UNIX,
-#endif
   G_SOCKET_FAMILY_IPV4 = GLIB_SYSDEF_AF_INET,
   G_SOCKET_FAMILY_IPV6 = GLIB_SYSDEF_AF_INET6
 } GSocketFamily;