git: use symbolic link

git: use symbolic link

The following error occurs when CONFIG_USE_APK is set.

```
ln /mnt/gl-mt6000-main/openwrt/build_dir/target-aarch64_cortex-a53_musl/git-2.46.2/.pkgdir/git/usr/lib/git-core/git /mnt/gl-mt6000-main/openwrt/build_dir/target-aarch64_cortex-a53_musl/git-2.46.2/.pkgdir/git/usr/bin/git
ln /mnt/gl-mt6000-main/openwrt/build_dir/target-aarch64_cortex-a53_musl/git-2.46.2/.pkgdir/git/usr/lib/git-core/git-shell /mnt/gl-mt6000-main/openwrt/build_dir/target-aarch64_cortex-a53_musl/git-2.46.2/.pkgdir/git/usr/bin/git-shell
touch /mnt/gl-mt6000-main/openwrt/build_dir/target-aarch64_cortex-a53_musl/git-2.46.2/.pkgdir/git.installed
mkdir -p /mnt/gl-mt6000-main/openwrt/staging_dir/target-aarch64_cortex-a53_musl/root-mediatek/stamp
SHELL= flock /mnt/gl-mt6000-main/openwrt/tmp/.root-copy.flock -c 'cp -fpR /mnt/gl-mt6000-main/openwrt/build_dir/target-aarch64_cortex-a53_musl/git-2.46.2/.pkgdir/git/. /mnt/gl-mt6000-main/openwrt/staging_dir/target-aarch64_cortex-a53_musl/root-mediatek/'
cp: '/mnt/gl-mt6000-main/openwrt/build_dir/target-aarch64_cortex-a53_musl/git-2.46.2/.pkgdir/git/./usr/bin/git' and '/mnt/gl-mt6000-main/openwrt/staging_dir/target-aarch64_cortex-a53_musl/root-mediatek/./usr/bin/git' are the same file
cp: '/mnt/gl-mt6000-main/openwrt/build_dir/target-aarch64_cortex-a53_musl/git-2.46.2/.pkgdir/git/./usr/bin/git-shell' and '/mnt/gl-mt6000-main/openwrt/staging_dir/target-aarch64_cortex-a53_musl/root-mediatek/./usr/bin/git-shell' are the same file
make[2]: *** [Makefile:167: /mnt/gl-mt6000-main/openwrt/staging_dir/target-aarch64_cortex-a53_musl/root-mediatek/stamp/.git_installed] Error 1
```
Use $(LN) instead of ln and use relative paths.

Signed-off-by: Hirokazu MORIKAWA <morikw2@gmail.com>