Add a nested include of <sys/linker_set.h> to make the sysctl(9) manpage accurate.  <sys/linker_set.h> is one of the very few headers similar to <sys/queue.h> for which nested includes is allowed.
Add a nested include of <sys/linker_set.h> to make the sysctl(9) manpage
accurate.  <sys/linker_set.h> is one of the very few headers similar to
<sys/queue.h> for which nested includes is allowed.

PR:		docs/153654
Reviewed by:	bde
MFC after:	1 month

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