selftests/bpf: Fix installation of urandom_read

For out-of-tree builds, $(TEST_CUSTOM_PROGS) require the $(OUTPUT)
prefix, otherwise the kselftest lib doesn't know how to install them:

rsync: [sender] link_stat "tools/testing/selftests/bpf/urandom_read" failed: No such file or directory (2)

Signed-off-by: Jean-Philippe Brucker <jean-philippe@linaro.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20210113163319.1516382-5-jean-philippe@linaro.org