bpf: Remove redundant slash

The trailing slash of LIBBPF_SRCS is redundant, remove it. Also inline
it as its only used in LIBBPF_INCLUDE.

Signed-off-by: Yuntao Wang <ytcoode@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20220305161013.361646-1-ytcoode@gmail.com