[OMPT] Fix use of 'DEBUG_PREFIX' in the OMPT headers

This is the only place that defines  this prefix in a header file and
was thus overriding and redefining other users of it. If we must use it
in a header file, at least repsect its old values.

Reviewed By: tianshilei1992

Differential Revision: https://reviews.llvm.org/D155316