Add TracingProcessor export to __init__.py (#192)

# Summary
This adds the missing TracingProcessor export to  __init__.py.

# Behavior
When trying to add a custom tracing processor, the TracingProcessor
importing fails with not found error when trying the example usage
proposed in issue #164

Specifically this line throws the error:
`add_trace_processor(MyTracingProcessor("output"))`

# Expected Behavior
Inspecting the init file, simply the import/export was missing. Adding
these made the example code work for me

# Test plan
Local dev of example code in #164

# Issue number
#164

# Checks
None

Author: rm-openai

src/agents/__init__.py

@@ -73,6 +73,7 @@
     SpanData,
     SpanError,
     Trace,
+    TracingProcessor,
     add_trace_processor,
     agent_span,
     custom_span,
@@ -208,6 +209,7 @@ def enable_verbose_stdout_logging():
     "set_tracing_disabled",
     "trace",
     "Trace",
+    "TracingProcessor",
     "SpanError",
     "Span",
     "SpanData",