gptfdisk: fix installation step

The installation step was not giving the full destination path even
though it was using -D, and it was forgetting to explicitly specify
the permissions of the file to install. This commit fixes both of
these minor issues.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>