Make the driver context union a defined type

Signed-off-by: Steven Cooreman <steven.cooreman@silabs.com>
diff --git a/include/psa/crypto_struct.h b/include/psa/crypto_struct.h
index 5d03d11..f22ed50 100644
--- a/include/psa/crypto_struct.h
+++ b/include/psa/crypto_struct.h
@@ -91,7 +91,7 @@
      * ID value zero means the context is not valid or not assigned to
      * any driver (i.e. none of the driver contexts are active). */
     unsigned int id;
-    union psa_driver_hash_context_u ctx;
+    psa_driver_hash_context_t ctx;
 };
 
 #define PSA_HASH_OPERATION_INIT {0, {0}}