bpf: minor fix to selftest test_stacktrace_build_id()

1. remove useless parameter list to ./urandom_read
2. add missing "\n" to the end of an error message

Fixes: 81f77fd0

 ("bpf: add selftest for stackmap with BPF_F_STACK_BUILD_ID")
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Song Liu <songliubraving@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>