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

package/boot/grub2/patches/100-musl-compat.patch

+--- a/grub-core/osdep/unix/hostdisk.c
++++ b/grub-core/osdep/unix/hostdisk.c
+@@ -48,11 +48,10 @@
+ #ifdef __linux__
+ # include <sys/ioctl.h>         /* ioctl */
+ # include <sys/mount.h>
+-# if !defined(__GLIBC__) || \
+-        ((__GLIBC__ < 2) || ((__GLIBC__ == 2) && (__GLIBC_MINOR__ < 1)))
++# if defined(__UCLIBC__)
+ /* Maybe libc doesn't have large file support.  */
+ #  include <linux/unistd.h>     /* _llseek */
+-# endif /* (GLIBC < 2) || ((__GLIBC__ == 2) && (__GLIBC_MINOR < 1)) */
++# endif /* __UCLIBC__ */
+ #endif /* __linux__ */
+ 
+ grub_uint64_t
+@@ -79,8 +78,7 @@ grub_util_get_fd_size (grub_util_fd_t fd
+   return st.st_size;
+ }
+ 
+-#if defined(__linux__) && (!defined(__GLIBC__) || \
+-        ((__GLIBC__ < 2) || ((__GLIBC__ == 2) && (__GLIBC_MINOR__ < 1))))
++#if defined(__linux__) && defined(__UCLIBC__)
+   /* Maybe libc doesn't have large file support.  */
+ int
+ grub_util_fd_seek (grub_util_fd_t fd, grub_uint64_t off)