Mark v1.5.x deprecated functions as deprecated

[embg]

We have a number of comments like

Note : this prototype will be marked as deprecated and generate compilation warnings on reaching v1.5.x

where the prototypes aren't actually marked as deprecated. For example:

zstd/lib/zstd.h

Lines 2445 to 2447 in 90597d7

	* Note : this prototype will be marked as deprecated and generate compilation warnings on reaching v1.5.x
	*/
	ZSTDLIB_STATIC_API size_t ZSTD_initDStream_usingDict(ZSTD_DStream* zds, const void* dict, size_t dictSize);

Filing an issue to remind myself to fix this.

[Cyan4973]

Yes, this is a good point @embg .
These symbols should probably be marked as deprecated now.