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

There is a reference to malloc() in #define __BITSET_ALLOC. Even
though this macro is only defined but not used, it causes the lang/gcc
ports to fail. The gcc ports "poison" a number of functions including
malloc() and prevent their use (including in macro definitions).

This commit moved the declaration of __BITSET_ALLOC into the
conditional block that depends on _KERNEL or _WANT_FREEBSD_BITSET
being defined.

There is no use of __BITSET_ALLOC in the FreeBSD sources, and userland
programs that want to use BITSEC_ALLOC will define _WANT_FREEBSD_BITSET
anyway.

This patch has been tested by building lang/gcc11 and a successful
make buildworld.

This commit shall be MFCed together with commit 5e04571cf3c.

MFC after: 1 month