bpf: Move the declaration of __bpf_obj_drop_impl() to bpf.h

both syscall.c and helpers.c have the declaration of
__bpf_obj_drop_impl(), so just move it to a common header file.

Signed-off-by: Hou Tao <houtao1@huawei.com>
Link: https://lore.kernel.org/r/20231020133202.4043247-6-houtao@huaweicloud.com


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