Make a constructor private.

(I'm looking into EnumMap stuff for other reasons, and I got to wondering if this was used outside the file. Nope.)

RELNOTES=n/a

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=233408580

guava-gwt/src-super/com/google/common/collect/super/com/google/common/collect/ImmutableEnumMap.java

@@ -36,7 +36,7 @@ static <K, V> ImmutableMap<K, V> asImmutable(Map<K, V> map) {
     return new ImmutableEnumMap<K, V>(map);
   }
 
-  ImmutableEnumMap(Map<? extends K, ? extends V> delegate) {
+  private ImmutableEnumMap(Map<? extends K, ? extends V> delegate) {
     super(WellBehavedMap.wrap(delegate));
   }
 }