lib/zstd/mem.h: replace __inline by inline

Currently, compiler_types.h #defines __inline as inline (and further
#defines inline to automatically attach some attributes), so this does
not change functionality. It serves as preparation for removing the
#define of __inline.

While at it, also remove the __attribute__((unused)) - it's already
included in the definition of the inline macro, and "open-coded"
__attribute__(()) should be avoided.

Since commit a95b37e2 (kbuild: get <linux/compiler_types.h> out of
<linux/kconfig.h>), compiler_types.h is automatically included by all
kernel C code - i.e., the definition of inline including the unused
attribute is guaranteed to be in effect whenever ZSTD_STATIC is
expanded.

Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Signed-off-by: Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>