libdtrace: Fix another uninitialized dtt_flags UB

Like r331073, eliminate a UB by fully initializing the struct with a designated
initializer. Note that the similar src_dtt is not fully used, so a similar
treatment was not absolutely required. I chose to leave it alone. It
wouldn't hurt to do the same thing, though.

Reported by: Coverity
Sponsored by: Dell EMC Isilon