Fix the format used by Kotlin/Native tasks for compiler plugin args

The correct format is "-P arg1", not "-Parg1".

libraries/tools/kotlin-gradle-plugin/src/main/kotlin/org/jetbrains/kotlin/gradle/tasks/KotlinNativeTasks.kt

@@ -236,7 +236,8 @@ open class KotlinNativeCompile : AbstractCompile() {
                 add("-Xplugin=$path")
             }
             compilerPluginOptions.arguments.forEach {
-                add("-P$it")
+                add("-P")
+                add(it)
             }
         }