Fix foward declaration duplication
diff --git a/tools/doxygen/Doxyfile.c++.internal b/tools/doxygen/Doxyfile.c++.internal
index 04e8f4e..52c406c 100644
--- a/tools/doxygen/Doxyfile.c++.internal
+++ b/tools/doxygen/Doxyfile.c++.internal
@@ -840,6 +840,7 @@
include/grpc/impl/codegen/atm_gcc_atomic.h \
include/grpc/impl/codegen/atm_gcc_sync.h \
include/grpc/impl/codegen/atm_windows.h \
+include/grpc/impl/codegen/exec_ctx_fwd.h \
include/grpc/impl/codegen/gpr_types.h \
include/grpc/impl/codegen/port_platform.h \
include/grpc/impl/codegen/slice.h \