Fix use of volatile

We need the pointer, A, to be volatile, to ensure the reads happen. bits
does not need to be volatile.

Signed-off-by: Janos Follath <janos.follath@arm.com>
diff --git a/library/bignum_core.h b/library/bignum_core.h
index 92c8d47..00c557b 100644
--- a/library/bignum_core.h
+++ b/library/bignum_core.h
@@ -665,7 +665,7 @@
  * \return        MBEDTLS_CT_FALSE if `A == 0`
  *                MBEDTLS_CT_TRUE  if `A != 0`.
  */
-mbedtls_ct_condition_t mbedtls_mpi_core_check_zero_ct(const mbedtls_mpi_uint *A,
+mbedtls_ct_condition_t mbedtls_mpi_core_check_zero_ct(volatile const mbedtls_mpi_uint *A,
                                                       size_t limbs);
 
 /**