Define _DTRACE_VERSION in sdt.h rather than setting it manually. This is
similar to what illumos does, and makes it easier to enable USDT probes
in third-party software that doesn't make use of the system makefiles.


git-svn-id: svn+ssh://svn.freebsd.org/base/head@270990 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f