bitstream: fix BIT_readBits and BIT_reloadDStream prototypes

[alex-moch]

Align the declarations of BIT_readBits() and BIT_reloadDStream() in bitstream.h with their FORCE_INLINE_TEMPLATE definitions.

The previous MEM_STATIC declarations caused an attribute mismatch between the header and the definitions, which can lead to incorrect compiler assumptions under certain toolchains and optimization levels.

---

  CC [M]  zstd/lib/common/entropy_common.o
In file included from /data/zstd/lib/common/fse.h:230,
                 from /data/zstd/lib/common/entropy_common.c:22:
/data/zstd/lib/common/bitstream.h:363:40: error: 'gnu_inline' attribute present on 'BIT_readBits'
  363 | FORCE_INLINE_TEMPLATE BitContainerType BIT_readBits(BIT_DStream_t* bitD, unsigned nbBits)
      |                                        ^~~~~~~~~~~~
/data/zstd/lib/common/bitstream.h:106:29: error: but not here
  106 | MEM_STATIC BitContainerType BIT_readBits(BIT_DStream_t* bitD, unsigned nbBits);
      |                             ^~~~~~~~~~~~
/data/zstd/lib/common/bitstream.h:413:42: error: 'gnu_inline' attribute present on 'BIT_reloadDStream'
  413 | FORCE_INLINE_TEMPLATE BIT_DStream_status BIT_reloadDStream(BIT_DStream_t* bitD)
      |                                          ^~~~~~~~~~~~~~~~~
/data/zstd/lib/common/bitstream.h:107:31: error: but not here
  107 | MEM_STATIC BIT_DStream_status BIT_reloadDStream(BIT_DStream_t* bitD);
      |                               ^~~~~~~~~~~~~~~~~