NIFI-7715 This closes apache#4456. Added IntelliJ generated .ipr to…

… `.gitignore`

- IntelliJ generates a `.ipr` Project File when exec the `mvn` goals
`mvn idea:idea` and `mvn idea:project`
  - [Maven `idea` Doc](https://maven.apache.org/plugins/maven-idea-plugin/usage.html)

Signed-off-by: ben <git@benjamin.ie>
Signed-off-by: Joe Witt <joewitt@apache.org>

.gitignore

@@ -12,6 +12,7 @@ nb-configuration.xml
 .idea/
 *.iml
 *.iws
+*.ipr
 *~
 
 .vscode/