Fix compilation of Enum on JDK 7

Change-Id: I2e58b6a527e4ef1fc7ed3359b8aea88d01a031fb

libcore: Better Enum fix for JDK 7

 * Jerome Pilliet suggests using a cast in the compareTo method instead of
   changing the field visibility. This is a much better fix.

Change-Id: I334105e0922225512a4d0a2d4b6c9c6ec71b9c51

Author: hyperb1iss

libdvm/src/main/java/java/lang/Enum.java

@@ -147,7 +147,7 @@ protected final Object clone() throws CloneNotSupportedException {
      * @see java.lang.Comparable
      */
     public final int compareTo(E o) {
-        return ordinal - o.ordinal;
+        return ordinal - ((Enum<?>) o).ordinal;
     }
 
     /**