Add missing @addtogroup tags.

These NDK docs weren't in any groups, so they don't show up in the
new site.

Test: https://irina-dot-devsite.googleplex.com/ndk/reference/group/native-activity
Test: https://irina-dot-devsite.googleplex.com/ndk/reference/group/networking
Test: https://irina-dot-devsite.googleplex.com/ndk/reference/group/tracing
Bug: http://b/77236573
Change-Id: Ic3154f77de1f13fe753b4c03537eab41b0a22848
diff --git a/include/android/trace.h b/include/android/trace.h
index d3b1fb6..d9e5d88 100644
--- a/include/android/trace.h
+++ b/include/android/trace.h
@@ -15,6 +15,11 @@
  */
 
 /**
+ * @addtogroup Tracing
+ * @{
+ */
+
+/**
  * @file trace.h
  * @brief Writes trace events to the system trace buffer.
  *
@@ -65,3 +70,5 @@
 #endif
 
 #endif // ANDROID_NATIVE_TRACE_H
+
+/** @} */