Fix compilation error when building static_lib (#9377)

Summary:
With memkind installed, either on a non-fb machine or using `ROCKSDB_NO_FBCODE=1`.

```
ROCKSDB_NO_FBCODE=1 make static_lib
```

Compilation failed due to unused variable warning treated as error. To bypass this, we need to
disable warning-as-error, which is not ideal.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/9377

Test Plan: Repeat the above command, and rely on CI.

Reviewed By: ajkr

Differential Revision: D33543343

Pulled By: riversand963

fbshipit-source-id: 9a2790b38c00b8696c7910287f4ae5a9b394341d

memory/memkind_kmem_allocator.h

@@ -22,6 +22,7 @@ class MemkindKmemAllocator : public BaseMemoryAllocator {
 
   static bool IsSupported(std::string* msg) {
 #ifdef MEMKIND
+    (void)msg;
     return true;
 #else
     *msg = "Not compiled with MemKind";