Add stub function for libor-trace.

OS X's ar(1) doesn't allow us to create an archive with no object files.
This patch adds a stub file with a stub function in it to make OS X
happy again.

src/trace/include.am

@@ -3,15 +3,17 @@
 AM_CPPFLAGS += -I$(srcdir)/src
 
 noinst_LIBRARIES += \
-  src/trace/libor-trace.a
-LIBOR_TRACE_A_SOURCES =
+	src/trace/libor-trace.a
+LIBOR_TRACE_A_SOURCES = \
+	src/trace/trace.c
 
 TRACEHEADERS = \
+	src/trace/trace.h \
 	src/trace/events.h
 
 if USE_EVENT_TRACING_DEBUG
 TRACEHEADERS += \
-  src/trace/debug.h
+	src/trace/debug.h
 endif
 
 # Library source files.

src/trace/trace.c

@@ -0,0 +1,11 @@
+/* Copyright (c) 2017, The Tor Project, Inc. */
+/* See LICENSE for licensing information */
+
+#include "trace.h"
+
+/** Initialize the tracing library. */
+void
+tor_trace_init(void)
+{
+}
+

src/trace/trace.h

@@ -0,0 +1,10 @@
+/* Copyright (c) 2017, The Tor Project, Inc. */
+/* See LICENSE for licensing information */
+
+#ifndef TOR_TRACE_TRACE_H
+#define TOR_TRACE_TRACE_H
+
+void tor_trace_init(void);
+
+#endif // TOR_TRACE_TRACE_H
+