Adjust struct layout for small size win

Signed-off-by: Dave Rodgman <dave.rodgman@arm.com>
diff --git a/include/mbedtls/sha256.h b/include/mbedtls/sha256.h
index 87e5cc6..87e259f 100644
--- a/include/mbedtls/sha256.h
+++ b/include/mbedtls/sha256.h
@@ -50,9 +50,9 @@
  *                 made in the call to mbedtls_sha256_starts().
  */
 typedef struct mbedtls_sha256_context {
+    unsigned char MBEDTLS_PRIVATE(buffer)[64];   /*!< The data block being processed. */
     uint32_t MBEDTLS_PRIVATE(total)[2];          /*!< The number of Bytes processed.  */
     uint32_t MBEDTLS_PRIVATE(state)[8];          /*!< The intermediate digest state.  */
-    unsigned char MBEDTLS_PRIVATE(buffer)[64];   /*!< The data block being processed. */
     int MBEDTLS_PRIVATE(is224);                  /*!< Determines which function to use:
                                                     0: Use SHA-256, or 1: Use SHA-224. */
 }