trivial: Fix a const-correctness issue discovered using clang
trivial: Fix a const-correctness issue discovered using clang

Fixes: https://github.com/hughsie/fwupd/issues/210

fwupd/fwupd,superm1/fwupd,superm1/fwupd,hughsie/fwupd,vathpela/fwupd,hughsie/fwupd,vathpela/fwupd,hughsie/fwupd,fwupd/fwupd,vathpela/fwupd,superm1/fwupd,fwupd/fwupd,vathpela/fwupd,hughsie/fwupd,fwupd/fwupd