selftests/bpf: Fix missing BUILD_BUG_ON() declaration

Explicitly include '<linux/build_bug.h>' to fix errors seen compiling with
gcc targeting mips64el/musl-libc:

  user_ringbuf.c: In function 'test_user_ringbuf_loop':
  user_ringbuf.c:426:9: error: implicit declaration of function 'BUILD_BUG_ON' [-Werror=implicit-function-declaration]
    426 |         BUILD_BUG_ON(total_samples <= c_max_entries);
        |         ^~~~~~~~~~~~
  cc1: all warnings being treated as errors

Fixes: e5a9df51 ("selftests/bpf: Add selftests validating the user ringbuf")
Signed-off-by: Tony Ambardar <tony.ambardar@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/b28575f9221ec54871c46a2e87612bb4bbf46ccd.1721713597.git.tony.ambardar@gmail.com