selftests/bpf: Fix liburandom_read.so linker error

When building with O=, the following linker error occurs:

    clang: error: no such file or directory: 'liburandom_read.so'

Fix by adding $(OUTPUT) to the linker search path.

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Link: https://lore.kernel.org/r/20230128000650.1516334-5-iii@linux.ibm.com


Signed-off-by: Alexei Starovoitov <ast@kernel.org>