Correct comment on the configuration option in x509.c

In x509.c, the self-test code is dependent on MBEDTLS_CERTS_C and
MBEDTLS_SHA256_C being enabled. At some point in the recent past that dependency
was on MBEDTLS_SHA1_C but changed to SHA256, but the comment wasn't updated.

This commit updates the comment.

Signed-off-by: Simon Butcher <[email protected]>

library/x509.c

@@ -1087,7 +1087,7 @@ cleanup:
     mbedtls_x509_crt_free( &clicert );
 #else
     ((void) verbose);
-#endif /* MBEDTLS_CERTS_C && MBEDTLS_SHA1_C */
+#endif /* MBEDTLS_CERTS_C && MBEDTLS_SHA256_C */
     return( ret );
 }