lz4_decompress: declare LZ4_decompress_safe_withPrefix64k static

Declare LZ4_decompress_safe_withPrefix64k as static to fix sparse
warning:

> warning: symbol 'LZ4_decompress_safe_withPrefix64k' was not declared.
> Should it be static?

Link: https://lkml.kernel.org/r/20210511154345.610569-1-thisisrast7@gmail.com
Signed-off-by: Rajat Asthana <thisisrast7@gmail.com>
Reviewed-by: Nick Terrell <terrelln@fb.com>
Cc: Gao Xiang <hsiangkao@redhat.com>
Cc: Joe Perches <joe@perches.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>