unix mem_info(): Dump GC info only if it's enabled.

unix/main.c

@@ -250,7 +250,9 @@ int usage(char **argv) {
 
 mp_obj_t mem_info(void) {
     printf("mem: total=%d, current=%d, peak=%d\n", m_get_total_bytes_allocated(), m_get_current_bytes_allocated(), m_get_peak_bytes_allocated());
+#if MICROPY_ENABLE_GC
     gc_dump_info();
+#endif
     return mp_const_none;
 }