Make mbedtls_mpi_core_ct_uint_table_lookup static

Now that we have a function that calls
mbedtls_mpi_core_ct_uint_table_lookup(), the compiler won't complain if
we make it static.

Signed-off-by: Janos Follath <janos.follath@arm.com>

library/bignum_core.c

@@ -540,6 +540,7 @@ cleanup:
     return( ret );
 }
 
+MBEDTLS_STATIC_TESTABLE
 void mbedtls_mpi_core_ct_uint_table_lookup( mbedtls_mpi_uint *dest,
                                             const mbedtls_mpi_uint *table,
                                             size_t limbs,