trivial: flashrom: Add SPDX license header
trivial: flashrom: Add SPDX license header

Clarify that the license was intended to be LGPL2.1 when the project
adjusted and this source file was just missed as well.

hughsie/fwupd,fwupd/fwupd,hughsie/fwupd,hughsie/fwupd,fwupd/fwupd,fwupd/fwupd,fwupd/fwupd,hughsie/fwupd