libbpf: Fix memory leak in strset

Free struct strset itself, not just its internal parts.

Fixes: 90d76d3e ("libbpf: Extract internal set-of-strings datastructure APIs")
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Link: https://lore.kernel.org/bpf/20211001185910.86492-1-andrii@kernel.org