macro: flip ONCE macro to make log_once() and friend actually log once

Previously, ONCE is false for the first time, and true for later times,
hence log_once() and log_once_errno() suppress logging in the first call,
rather than later calls.

Fortunately, ONCE macro is only used in log_once() and log_once_errno(),
hence this only fixes spurious logging.

src/test/test-macro.c

@@ -1102,4 +1102,20 @@ TEST(u64_multiply_safe) {
         assert_se(u64_multiply_safe(UINT64_MAX, UINT64_MAX) == 0);
 }
 
+static void test_once_impl(void) {
+        static unsigned count = 0;
+
+        if (ONCE) {
+                log_info("This should be logged only once.");
+                count++;
+        }
+
+        ASSERT_EQ(count, 1u);
+}
+
+TEST(once) {
+        for (unsigned i = 0; i < 20; i++)
+                test_once_impl();
+}
+
 DEFINE_TEST_MAIN(LOG_INFO);