Mark entrypointFunctions as used for clang+lto

LLVM currently ignores inline assembly references to symbols during
dead code and data elimination while linking with link-time optimizations
enabled.

This manifests in a very large amount of unresolved references to
entrypointFunctions when building libglvnd using clang with LTO enabled.

This issue fixes #220 by marking the entrypointFunctions array as used,
using a compiler attribute.

Signed-off-by: Charlotte Delenk <darkkirb@darkkirb.de>
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