arm: Add a placeholder file in lieu of the former `filter_neon.S`

In the previous commit 9e538750d99c8f1accf7e93878e4fde47c069908
we removed the obsolete assembler implementation `filter_neon.S`.

In this commit we add a stand-in for the original file, restoring
the original source tree structure, for the benefit of continuing
hassle-free libpng source upgrades in the 1.6.x line.
1 file changed