grub2: fix build against musl

Invert the `__GLIBC__` guards into `!__UCLIBC__` ones to avoid
invalid references to `_llseek` on musl.

Signed-off-by: Jo-Philipp Wich <jow@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@46125 3c298f89-4303-0410-b956-a3cf2f4a3e73