selftests: bpf: test_progs: initialize duration in singal_pending test

CHECK macro implicitly uses duration. We call CHECK() a couple of times
before duration is initialized from bpf_prog_test_run().
Explicitly set duration to 0 to avoid compiler warnings.

Fixes: 740f8a6572 ("selftests/bpf: make sure signal interrupts BPF_PROG_TEST_RUN")
Signed-off-by: Stanislav Fomichev <sdf@google.com>
Acked-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

tools/testing/selftests/bpf/prog_tests/signal_pending.c

@@ -12,7 +12,7 @@ static void test_signal_pending_by_type(enum bpf_prog_type prog_type)
 	struct itimerval timeo = {
 		.it_value.tv_usec = 100000, /* 100ms */
 	};
-	__u32 duration, retval;
+	__u32 duration = 0, retval;
 	int prog_fd;
 	int err;
 	int i;