package/gptfdisk: fix runtime failure with popt 1.19

Fix the following runtime failure raised since bump of popt to version
1.19 in commit 895bfba9:

Problem opening  for reading! Error is 2.
The specified file does not exist!

Fixes:
 - No autobuilder failure

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Reported-by: Florian Fainelli <f.fainelli@gmail.com>
Tested-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>