Fix build breakage for non-CR50 builds. Commit ae8d044cd1 introduced a externally defined callback _plat__ResetCallback() which is not available outside the CR50 environment. Address the issue with conditional compilation. TEST=docker fuzz container builds; CR50 builds Change-Id: If23c036c41a900bbcedd4b0bfd9edb1e89c17a0f Reviewed-on: https://chromium-review.googlesource.com/822601 Commit-Ready: Nagendra Modadugu <[email protected]> Tested-by: Nagendra Modadugu <[email protected]> Reviewed-by: Vadim Bendebury <[email protected]>