Update tensorflow submodule (#2143)

Update tensorflow submodule to update nnapi_delegate and NeuralNetworksEx.h

Signed-off-by: Hyeongseok Oh <hseok82.oh@samsung.com>

diff --git a/externals/tensorflow b/externals/tensorflow
index dc9fc1b..fd70f51 160000 (submodule)
--- a/externals/tensorflow
+++ b/externals/tensorflow
@@ -1 +1 @@
-Subproject commit dc9fc1bff1e381f395a157b8ea1e7762c39f5c58
+Subproject commit fd70f515599ee5d357e9beda63a5b161abaf12b1