Fix foward declaration duplication
diff --git a/tools/doxygen/Doxyfile.c++ b/tools/doxygen/Doxyfile.c++
index ff3a0e3..62d6e79 100644
--- a/tools/doxygen/Doxyfile.c++
+++ b/tools/doxygen/Doxyfile.c++
@@ -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 \
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 \
diff --git a/tools/doxygen/Doxyfile.core b/tools/doxygen/Doxyfile.core
index 1e748ba..4a1378e 100644
--- a/tools/doxygen/Doxyfile.core
+++ b/tools/doxygen/Doxyfile.core
@@ -779,6 +779,7 @@
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 \
@@ -818,6 +819,7 @@
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 \
diff --git a/tools/doxygen/Doxyfile.core.internal b/tools/doxygen/Doxyfile.core.internal
index 55b8d40..2be2468 100644
--- a/tools/doxygen/Doxyfile.core.internal
+++ b/tools/doxygen/Doxyfile.core.internal
@@ -779,6 +779,7 @@
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 \
@@ -1208,6 +1209,7 @@
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 \