bpf: Add bpf_rbtree_{add,remove,first} decls to bpf_experimental.h

These kfuncs will be used by selftests in following patches

Signed-off-by: Dave Marchevsky <davemarchevsky@fb.com>
Link: https://lore.kernel.org/r/20230214004017.2534011-7-davemarchevsky@fb.com


Signed-off-by: Alexei Starovoitov <ast@kernel.org>