KAPT: Kapt generates illegal stubs for private interface methods

Interface methods that were private got both the `default` and `private`
modifiers when using `jvm-default=all` which is not valid.

The stub will now just contain `private` in this case.

This fixes KT-48013.

plugins/kapt3/kapt3-compiler/testData/converter/jvmDefaultAll.kt

@@ -1,4 +1,5 @@
 // !JVM_DEFAULT_MODE: all
+// EXPECTED_ERROR: (kotlin:15:5) modifier private not allowed here
 
 interface Foo {
     fun foo() {
@@ -10,4 +11,8 @@ interface Foo {
     }
 
     fun bar()
+
+    private fun privateMethodWithDefault() {
+        System.out.println("privateMethodWithDefault")
+    }
 }