selftests/bpf: Fix memory leak in extract_build_id()

getline() allocates string, which has to be freed.

Fixes: 81f77fd0

 ("bpf: add selftest for stackmap with BPF_F_STACK_BUILD_ID")
Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Cc: Song Liu <songliubraving@fb.com>
Link: https://lore.kernel.org/bpf/20200429012111.277390-7-andriin@fb.com