selftests/bpf: Add liburandom_read.so to TEST_GEN_FILES

Added urandom_read shared lib is missing from the list of installed
files what makes urandom_read test after `make install` or `make
gen_tar` broken.

Add the library to TEST_GEN_FILES. The names in the list do not
contain $(OUTPUT) since it's added by lib.mk code.

Fixes: 00a0fa2d ("selftests/bpf: Add urandom_read shared lib and USDTs")
Signed-off-by: Yauheni Kaliuta <ykaliuta@redhat.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20220920161409.129953-1-ykaliuta@redhat.com