Add \PDO::SQLITE_ATTR_READONLY_STATEMENT

This attribute is a boolean value. It is taken from the return value of
sqlite3_stmt_readonly(), indicating if and only if the prepared statement makes
no direct changes to the content of the database.

ext/pdo_sqlite/php_pdo_sqlite_int.h

@@ -76,6 +76,7 @@ extern const struct pdo_stmt_methods sqlite_stmt_methods;
 
 enum {
 	PDO_SQLITE_ATTR_OPEN_FLAGS = PDO_ATTR_DRIVER_SPECIFIC,
+	PDO_SQLITE_ATTR_READONLY_STATEMENT
 };
 
 #endif