gptfdisk: fix static linking issue with iconv

Fixes:
http://autobuild.buildroot.net/results/e89/e891a5bab72690a530781ad87d5804a53a209b5c/



Similar to libintl, some of the gptfdisk dependencies may link against
libiconv, so we need to do so as well when linking statically.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>