Restore MixinDefinition.getOuterMixinDefinition()

Got lost with the changes for the object literals.
Used to be called `getOuter()`, new name is a bit more specific.

Signed-off-by: Stefan Marr <git@stefan-marr.de>

src/som/compiler/MixinDefinition.java

@@ -132,6 +132,13 @@ public SSymbol getName() {
     return name;
   }
 
+  /**
+   * Used by the SOMns Language Server.
+   */
+  public MixinDefinition getOuterMixinDefinition() {
+    return instanceScope.getOuterMixin().getMixinDefinition();
+  }
+
   public SSymbol getPrimaryFactorySelector() {
     return primaryFactoryName;
   }