libgd: add conflicts to each other

[BKPepe]

Maintainer: @jow- introduced by @flyn-org in 0762c72
Compile tested: N/A
Run tested: N/A

The full variant should conflict with the default variant. This prevents that
libgd and libgd-full could be installed side by side, and also, the full
variant should provide the libgd. Otherwise, if you install libgd-full,
you can not install vnstat.

Fixes: https://forum.turris.cz/t/updater-okpg-collision-of-dependencies/17520/3

[flyn-org]

Looks good. Recommend merge.

[hnyman]

This caused dependency conflict.

perus@ub2204:/Openwrt/r7800$ make defconfig
Collecting package info: done
tmp/.config-package.in:59546:warning: leading whitespace ignored
tmp/.config-package.in:53246:error: recursive dependency detected!
tmp/.config-package.in:53246:	symbol PACKAGE_libgd is selected by PACKAGE_libgd-full
tmp/.config-package.in:53280:	symbol PACKAGE_libgd-full depends on PACKAGE_libgd
For a resolution refer to Documentation/kbuild/kconfig-language.rst
subsection "Kconfig recursive dependency limitations"

The basic/default variant should have a different name than the full variant's PROVIDES.

e.g. libgd-basic and libgd-full, and both provide libgd, and libgd-full conflicts with libgd-basic.

Ps.

Compile tested: N/A

Even running make defconfig should have revealed the created config conflict.