Move ZSTD_DEPRECATED before ZSTDLIB_API/ZSTDLIB_STATIC_API

[MaskRay]

Clang doesn't allow [[deprecated(...)]] attribute after __attribute__. Move [[deprecated(...)]] before __attribute__ to fix C++14/C++17 uses with Clang.

Fix #3250

---

The error looks like:

/tmp/opt/zstd/include/zstd.h:191:1: error: an attribute list cannot appear here
ZSTD_DEPRECATED("Replaced by ZSTD_getFrameContentSize")
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/tmp/opt/zstd/include/zstd.h:49:38: note: expanded from macro 'ZSTD_DEPRECATED'
#    define ZSTD_DEPRECATED(message) [[deprecated(message)]]
                                     ^~~~~~~~~~~~~~~~~~~~~~~

[Cyan4973]

This change incorrectly makes ZSTD_compressBegin_usingCDict() deprecated, but it's not.

The deprecation warning was meant for the next symbol, ZSTD_copyCCtx().

#3274 is being pushed, in the hope that it helps clarity.

[MaskRay]

All tests passed :)