unix/main: mp_verbose_flag available only if MICROPY_DEBUG_PRINTERS is true.

Not available for minimal build for example.

unix/main.c

@@ -524,8 +524,10 @@ int main(int argc, char **argv) {
                 break;
             } else if (strcmp(argv[a], "-X") == 0) {
                 a += 1;
+            #if MICROPY_DEBUG_PRINTERS
             } else if (strcmp(argv[a], "-v") == 0) {
                 mp_verbose_flag++;
+            #endif
             } else if (strncmp(argv[a], "-O", 2) == 0) {
                 if (unichar_isdigit(argv[a][2])) {
                     MP_STATE_VM(mp_optimise_value) = argv[a][2] & 0xf;