sys/_bitset.h: Fix fall-out from commit 5e04571cf3c

The changes to the bitset macros allowed sched.h to be included
into userland programs without name space pollution due to BIT_*
and BITSET_* macros.

The definition of a "struct bitset" had been overlooked. This name
space pollution caused the build of port print/miktex to fail.

This commit makes the definition of struct bitset depend on the
same condition as the visibility of the BIT_* and BITSET_* macros,
i.e. needs _KERNEL or _WANT_FREEBSD_BITSET to be defined before
including sys/_bitset.h.

It has been tested with "make universe" since a prior attempt to
fix the issue broke the PowerPC64 kernel build.

This commit shall be MFCed together with commit 5e04571cf3c.

Reported by: arrowd
MFC after: 1 month