grub2: opt-out of gc-sections usage

grub2: opt-out of gc-sections usage

This fixes building with USE_GC_SECTIONS enabled:

./staging_dir/toolchain-x86_64_gcc-12.3.0_musl/bin/../lib/gcc/x86_64-openwrt-linux-musl/12.3.0/../../../../x86_64-openwrt-linux-musl/bin/ld.bfd: --gc-sections requires a defined symbol root specified by -e or -u
collect2: error: ld returned 1 exit status
make[4]: *** [Makefile:27665: disk.module] Error 1
make[4]: Leaving directory './build_dir/target-x86_64_musl/linux-x86_64/grub-efi/grub-2.06/grub-core'
make[3]: *** [Makefile:26916: all] Error 2
make[3]: Leaving directory './build_dir/target-x86_64_musl/linux-x86_64/grub-efi/grub-2.06/grub-core'
make[2]: *** [Makefile:205: ./build_dir/target-x86_64_musl/linux-x86_64/grub-efi/grub-2.06/.built] Error 2
make[2]: Leaving directory './package/boot/grub2'
time: package/boot/grub2/efi/compile#3.21#1.34#4.28
    ERROR: package/boot/grub2 failed to build (build variant: efi).

Signed-off-by: Javier Marcet <javier@marcet.info>