build: fix dtrace in FreeBSD build (#869)

Addresses joyent/libuv#869.

FreeBSD make only sets `$<` (aka `.IMPSRC`) in suffix-transformation rules.
Changed the syntax in the recipe for the rule to make
`include/uv-dtrace.h` to explicitly name `src/unix/uv-dtrace.d` instead of
using `$<`.