improve sql parser for informix skip/first clause.

src/main/java/com/alibaba/druid/sql/ast/statement/SQLSelectQueryBlock.java

@@ -38,6 +38,7 @@ public class SQLSelectQueryBlock extends SQLObjectImpl implements SQLSelectQuery
     protected boolean                   noWait        = false;
     protected SQLExpr                   waitTime;
     protected SQLExpr                   first;
+    protected SQLExpr                   offset;
 
     public SQLSelectQueryBlock(){
 
@@ -161,6 +162,17 @@ public class SQLSelectQueryBlock extends SQLObjectImpl implements SQLSelectQuery
         this.first = first;
     }
 
+    public SQLExpr getOffset() {
+        return offset;
+    }
+
+    public void setOffset(SQLExpr offset) {
+        if (offset != null) {
+            offset.setParent(this);
+        }
+        this.offset = offset;
+    }
+
 	@Override
     protected void accept0(SQLASTVisitor visitor) {
         if (visitor.visit(this)) {