gptfdisk: fix incorrect TARGET on non-linux host

gptfdisk: fix incorrect TARGET on non-linux host

Fixed by defining TARGET=linux, as suggested in the README.

By default TARGET is derived from the host using uname -s,
which produces build errors on a non-linux hosts:

TARGET is not set; trying to determine target based on host OS....
Detected OS is Darwin
Build target is macos
...
x86_64-openwrt-linux-musl-g++: error: unrecognized command-line option '-arch'

Helped-by: Tianling Shen <cnsztl@immortalwrt.org>
Signed-off-by: Georgi Valkov <gvalkov@gmail.com>