libbpf: Fix whitespace in btf_add_composite() comment

mainline inclusion
from mainline-5.13-rc1
commit 1b1ce92b
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I5EUVD
CVE: NA

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=1b1ce92b24331b569a444858fc487a1ca19dc778

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Remove trailing space.
Signed-off-by: NIlya Leoshkevich <iii@linux.ibm.com>
Signed-off-by: NAlexei Starovoitov <ast@kernel.org>
Acked-by: NYonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20210226202256.116518-3-iii@linux.ibm.com
(cherry picked from commit 1b1ce92b)
Signed-off-by: NWang Yufen <wangyufen@huawei.com>