<net/sff8472.h>: Conditionally export table of ID names

Only export the array of ID names if either _WANT_SFF_8024_ID or
_WANT_SFF_8472_ID is defined. Exporting them unconditionally can
trigger unused variable warnings if a consumer doesn't use the array.

Reviewed by: olce, bz, brooks
Differential Revision: https://reviews.freebsd.org/D49955