[K/N] Fix benchmark build

The bootstrap advance to 1.8.20-dev-2904 broke the K/N performance
benchmark build, when built in non-Java8 environments.


Co-authored-by: Troels Lund <troels@google.com>

Merge-request: KOTLIN-MR-576
Merged-by: Pavel Punegov <pavel.punegov@jetbrains.com>

kotlin-native/performance/buildSrc/build.gradle.kts

@@ -136,4 +136,7 @@ afterEvaluate {
             apiVersion = "1.4"
         }
     }
+    tasks.withType<JavaCompile> {
+        targetCompatibility = "1.8"
+    }
 }