fix prach example and close rf at exit

lib/src/phy/phch/test/prach_test_usrp.c

@@ -232,6 +232,7 @@ int main(int argc, char **argv) {
   
   srslte_vec_save_file(output_filename,buffer,11*flen*sizeof(cf_t));
 
+  srslte_rf_close(&rf);
   srslte_prach_free(p);
   free(p);