gettext-flatpak: Fix only the last pot is updated

[ryonakano]

Fixes only the last pot file is updated when there are multiple pot files.

ryo@lb450m:~/work/ryonakano/pinit (test-gettext-flatpak)$ TRANSLATION_FILES=$(git ls-files | grep \.pot$)
ryo@lb450m:~/work/ryonakano/pinit (test-gettext-flatpak)$ echo $TRANSLATION_FILES
po/com.github.ryonakano.pinit.pot po/extra/extra.pot
ryo@lb450m:~/work/ryonakano/pinit (test-gettext-flatpak)$ GETTEXT_TARGETS=$(echo $TRANSLATION_FILES | sed 's/.*\///' | sed 's/.pot/-pot/')
ryo@lb450m:~/work/ryonakano/pinit (test-gettext-flatpak)$ echo $GETTEXT_TARGETS 
extra-pot
ryo@lb450m:~/work/ryonakano/pinit (test-gettext-flatpak)$