Re-add memory_allocator.h include from cache.h (#11229)

Summary:
Enough users of NewJemallocNodumpAllocator() with cache.h to justify keeping it. (Reverting one little part of https://github.com/facebook/rocksdb/issues/11192)

Pull Request resolved: https://github.com/facebook/rocksdb/pull/11229

Test Plan: CI

Reviewed By: ajkr

Differential Revision: D43337140

Pulled By: pdillinger

fbshipit-source-id: 886b27b96b395619a4209f51b9b7787f4fe89e57

include/rocksdb/cache.h

@@ -16,12 +16,12 @@
 #include <string>
 
 #include "rocksdb/compression_type.h"
+#include "rocksdb/memory_allocator.h"
 
 namespace ROCKSDB_NAMESPACE {
 
 class Cache;  // defined in advanced_cache.h
 struct ConfigOptions;
-class MemoryAllocator;
 class SecondaryCache;
 
 // Classifications of block cache entries.