GH-48566: [C++][CI] Fix compilation on Valgrind job

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"