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

tools/testing/selftests/bpf/Makefile

@@ -82,7 +82,7 @@ TEST_GEN_PROGS_EXTENDED = test_sock_addr test_skb_cgroup_id_user \
 	test_lirc_mode2_user xdping test_cpp runqslower bench bpf_testmod.ko \
 	xdpxceiver
 
-TEST_CUSTOM_PROGS = urandom_read
+TEST_CUSTOM_PROGS = $(OUTPUT)/urandom_read
 
 # Emit succinct information message describing current building step
 # $1 - generic step name (e.g., CC, LINK, etc);