Fix not exporting encode_png on windows

[alanb-sony]

As encode_png.h was not included encode_png in encode_png.cpp becomes the definition of a new function not the definition of the function declared in encode_png.h, therefore it is not decorated with C10_EXPORT and is not exported to the DLL on Windows.

[pytorch-bot]

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🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/vision/9486

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✅ You can merge normally! (2 Unrelated Failures)

As of commit 08b1e2a with merge base 499ca51 ():

BROKEN TRUNK - The following jobs failed but were present on the merge base:

👉 Rebase onto the `viable/strict` branch to avoid these failures

• Tests / unittests-macos (3.10, macos-m1-stable) / macos-job (gh) (trunk failure)
  test/test_transforms_v2.py::TestErase::test_transform_image_correctness[4-cpu-dtype1-value-random]
• Tests / unittests-macos (3.14, macos-m1-stable) / macos-job (gh) (trunk failure)
  test/test_transforms_v2.py::TestErase::test_transform_image_correctness[4-cpu-dtype1-value-random]

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[NicolasHug]

Thanks for the fix @alanb-sony , do you mind addressing the CLA comment #9486 (comment) so we can merge this?