Fix LRU debugger display on net standard (#458)

Author: bitfaster

BitFaster.Caching/Lru/ConcurrentLruCore.cs

@@ -826,7 +826,11 @@ private static CachePolicy CreatePolicy(ConcurrentLruCore<K, V, I, P, T> lru)
         // it becomes immutable. However, this object is then somewhere else on the 
         // heap, which slows down the policies with hit counter logic in benchmarks. Likely
         // this approach keeps the structs data members in the same CPU cache line as the LRU.
+#if NETCOREAPP3_0_OR_GREATER
         [DebuggerDisplay("Hit = {Hits}, Miss = {Misses}, Upd = {Updated}, Evict = {Evicted}")]
+#else
+        [DebuggerDisplay("Hit = {Hits}, Miss = {Misses}, Evict = {Evicted}")]
+#endif
         private class Proxy : ICacheMetrics, ICacheEvents<K, V>, IBoundedPolicy, ITimePolicy
         {
             private readonly ConcurrentLruCore<K, V, I, P, T> lru;