Change the NS_PER_SEC (and friends) from enum to static const

New version of clang (19) has introduced a stricter checks when mixing
integer (and float types) with enums.  In this case, we used enum {}
as C17 doesn't have constexpr yet.  Change the time conversion constants
to be static const unsigned int instead of enum values.

tests/libtest/dns.c

@@ -203,8 +203,8 @@ void
 dns_test_nap(uint32_t usec) {
 	struct timespec ts;
 
-	ts.tv_sec = usec / 1000000;
-	ts.tv_nsec = (usec % 1000000) * 1000;
+	ts.tv_sec = usec / (long)US_PER_SEC;
+	ts.tv_nsec = (usec % (long)US_PER_SEC) * (long)NS_PER_US;
 	nanosleep(&ts, NULL);
 }