libbpf: Fix memory leak when emitting final btf_ext

Free temporary allocated memory used to construct finalized .BTF.ext data.
Found by Coverity static analysis on libbpf's Github repo.

Fixes: 8fd27bf6 ("libbpf: Add BPF static linker BTF and BTF.ext support")
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Song Liu <songliubraving@fb.com>
Link: https://lore.kernel.org/bpf/20210327042502.969745-1-andrii@kernel.org