Add token for require_once

Author: Jeckerson

parser/zephir.lemon

@@ -41,6 +41,7 @@
 %left INTERNAL PUBLIC PROTECTED STATIC PRIVATE SCOPED .
 %left COMMA .
 %right REQUIRE .
+%right REQUIRE_ONCE .
 %right DOUBLEARROW .
 %right QUESTION .
 %right LIKELY UNLIKELY .