[LIBS] rbtree: Fix compilation warnings for arm64 host

The aarch64 (or arm64) complains on usage of __always_inline (i.e.
force inline attribute) for rbtree functions (don't know why?).

To take care of the warnings, we simply replace __always_inline
with inline.

Signed-off-by: Anup Patel <anup@brainfault.org>