nlist.3: Add stab(5) Xref

Commit 876a17321c removed the a.out Xref as nlist(3) no longer
supports a.out, but this left nlist(3) without a reference to a page
documenting struct nlist.

struct nlist is documented in both a.out(5) and stab(5), so add an Xref
to the latter.

Reported by:	brooks
Fixes: 876a17321c ("nlist.3: Replace a.out(5) Xref with elf(5)")

lib/libc/gen/nlist.3

@@ -72,7 +72,8 @@ if the file
 .Fa filename
 does not exist or is not executable, the returned value is \-1.
 .Sh SEE ALSO
-.Xr elf 5
+.Xr elf 5 ,
+.Xr stab 5
 .Sh HISTORY
 A
 .Fn nlist