Fix creating static ktx_read convenience lib

[dg0yt]

Fixes "duplicate member name" warnings:

[130/148] : [...] libtool -static -o /Users/vcpkg/Data/b/ktx/x64-osx-rel/libktx_read.a /Users/vcpkg/Data/b/ktx/x64-osx-rel/libktx.a /Users/vcpkg/Data/b/ktx/x64-osx-rel/external/astc-encoder/Source/libastcenc-avx2-static.a
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/libtool: warning for architecture: x86_64h duplicate member name 'astcenc_averages_and_directions.cpp.o' from '/Users/vcpkg/Data/b/ktx/x64-osx-rel/external/astc-encoder/Source/libastcenc-avx2-static.a(astcenc_averages_and_directions.cpp.o)' and '/Users/vcpkg/Data/b/ktx/x64-osx-rel/libktx.a(astcenc_averages_and_directions.cpp.o)'

[CLAassistant]

All committers have signed the CLA.

[dg0yt]

New CLA? It was green in KhronosGroup/Vulkan-ExtensionLayer#436 (comment).

[MarkCallow]

New CLA? It was green in KhronosGroup/Vulkan-ExtensionLayer#436 (comment).

I have given up trying to understand the vagaries of Khronos's CLAassistant. Please just sign it again.

[dg0yt]

Maybe close/re-open helps?

[MarkCallow]

This PR is being hit by an astonishing number of network errors. Now cloudflare Tokyo is broken when I try to access the Vulkan SDK to investigate the latest failure.

[MarkCallow]

@dg0yt Are you aware of the vcpkg-related work @kring is doing? He submitted PR #1075 just after I had started work on updating the way we build basis_universal. That deeply affected the PR so I closed it asked him to re-submit. I haven't heard anything since.

[dg0yt]

No. Submitting PRs to vcpkg would catch my attention ;-)