Add UNIMPLEMENTED() macro

This was also lost in the previous git rebase error for the sockets API
change.

Change-Id: I9d0e2b52dab573eccf374587d62c3d928c53f57d
Reviewed-on: https://chromium-review.googlesource.com/1151056
Reviewed-by: Brandon Tolsch <btolsch@chromium.org>
diff --git a/platform/api/logging.h b/platform/api/logging.h
index d6d3589..cd95290 100644
--- a/platform/api/logging.h
+++ b/platform/api/logging.h
@@ -127,6 +127,8 @@
 #define DVLOG(l) VLOG_IF(l, DCHECK_IS_ON())
 #define DVLOG_IF(l, condition) VLOG_IF(l, DCHECK_IS_ON() && (condition))
 
+#define UNIMPLEMENTED() LOG_ERROR << __func__ << ": unimplemented"
+
 }  // namespace platform
 }  // namespace openscreen