trivial: Fix a warning when compiling on Windows
trivial: Fix a warning when compiling on Windows

Move the function prototype out of the GUnixInputStream-using ones.

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