Enable -Wextra

-Wsign-compare has been disabled because flex generates a comparison
that triggers a warning and cannot be fixed in the code.

Signed-off-by: Antonio Niño Díaz <antonio_nd@outlook.com>

src/gfx/main.c

@@ -53,6 +53,7 @@ int main(int argc, char *argv[])
 			break;
 		case 'F':
 			opts.hardfix = true;
+			/* fallthrough */
 		case 'f':
 			opts.fix = true;
 			break;