tracing: add DEFINE_TRACE_FMT to tracepoint.h

This patch creates a DEFINE_TRACE_FMT to map to DECLARE_TRACE.
This allows for the developers to place format strings and
args in with their tracepoint declaration. A tracer may now
override the DEFINE_TRACE_FMT macro and use it to record
a default format.

Signed-off-by: Steven Rostedt <srostedt@redhat.com>