render_hkl: Accept the -o option

src/render_hkl.c

@@ -474,7 +474,7 @@ int main(int argc, char *argv[])
 	};
 
 	/* Short options */
-	while ((c = getopt_long(argc, argv, "hj:p:w:c:y:d:r:",
+	while ((c = getopt_long(argc, argv, "hj:p:w:c:y:d:r:o:",
 	                        longopts, NULL)) != -1) {
 
 		switch (c) {