selftests: bpf: fix urandom_read build issue

gcc complains that urandom_read gets built twice.

gcc -o tools/testing/selftests/bpf/urandom_read
-static urandom_read.c -Wl,--build-id
gcc -Wall -O2 -I../../../include/uapi -I../../../lib -I../../../lib/bpf
-I../../../../include/generated  -I../../../include    urandom_read.c
urandom_read -lcap -lelf -lrt -lpthread -o
tools/testing/selftests/bpf/urandom_read
gcc: fatal error: input file
‘tools/testing/selftests/bpf/urandom_read’ is the
same as output file
compilation terminated.
../lib.mk:110: recipe for target
'tools/testing/selftests/bpf/urandom_read' failed
To fix this issue remove the urandom_read target and so target
TEST_CUSTOM_PROGS gets used.

Fixes: 81f77fd0 ("bpf: add selftest for stackmap with BPF_F_STACK_BUILD_ID")
Signed-off-by: Anders Roxell <anders.roxell@linaro.org>
Acked-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>