unix/moduselect: Don't allow both posix and non-posix configurations.

Signed-off-by: Jim Mussared <jim.mussared@gmail.com>

ports/unix/moduselect.c

@@ -29,6 +29,10 @@
 
 #if MICROPY_PY_USELECT_POSIX
 
+#if MICROPY_PY_USELECT
+#error "Can't have both MICROPY_PY_USELECT and MICROPY_PY_USELECT_POSIX."
+#endif
+
 #include <stdio.h>
 #include <errno.h>
 #include <poll.h>