Properly specify dependences on alarm.h in build.yaml
diff --git a/tools/doxygen/Doxyfile.c++.internal b/tools/doxygen/Doxyfile.c++.internal
index 6135463..e0650d7 100644
--- a/tools/doxygen/Doxyfile.c++.internal
+++ b/tools/doxygen/Doxyfile.c++.internal
@@ -760,7 +760,8 @@
 # spaces.
 # Note: If this tag is empty the current directory is searched.
 
-INPUT                  = include/grpc++/channel.h \
+INPUT                  = include/grpc++/alarm.h \
+include/grpc++/channel.h \
 include/grpc++/client_context.h \
 include/grpc++/completion_queue.h \
 include/grpc++/create_channel.h \