GH-48566: [C++][CI] Fix compilation on Valgrind job (#48567)

### Rationale for this change

A `#include` required on Valgrind (due to conditional compilation) was removed in PR #45462.

### Are these changes tested?

Yes, by existing CI jobs.

### Are there any user-facing changes?

No.
* GitHub Issue: #48566

Authored-by: Antoine Pitrou <[email protected]>
Signed-off-by: Antoine Pitrou <[email protected]>

Author: pitrou

cpp/src/arrow/dataset/file_parquet_encryption_test.cc

@@ -38,6 +38,7 @@
 #include "arrow/util/thread_pool.h"
 #include "parquet/arrow/reader.h"
 #include "parquet/encryption/crypto_factory.h"
+#include "parquet/encryption/encryption_internal.h"  // for EnsureBackendInitialized
 #include "parquet/encryption/kms_client.h"
 #include "parquet/encryption/test_in_memory_kms.h"