If you are using maven project then you just need to add the jstl-1.2 jar in your dependency. If you are simply adding the jar to your project then it's possible that jar file is not added in your project project artifact. You simply need to add the jar file in WEB-INF/lib file.
This is how your project should look when jstl is not added to the artifact. Just hit the fix button and intellij will add the jar file in the above mentioned path. Run the project and bang.