grub2: add patch to fix build with recent gcc versions

grub2: add patch to fix build with recent gcc versions

This commit backports a patch from upstream grub2 that fixes a build
issue occuring at least with recent gcc versions:

gettext/gettext.c:37:36: error: storage size of 'main_context' isn't known
 static struct grub_gettext_context main_context, secondary_context;

Fixes bug #8991.

Bug reproduced with:

BR2_x86_64=y
BR2_PACKAGE_HOST_LINUX_HEADERS_CUSTOM_4_5=y
BR2_TOOLCHAIN_BUILDROOT_GLIBC=y
BR2_GCC_VERSION_6_X=y
BR2_TARGET_GRUB2=y
BR2_TARGET_GRUB2_X86_64_EFI=y
BR2_TARGET_GRUB2_BUILTIN_MODULES="boot linux ext2 fat squash4 part_msdos part_gpt normal efi_gop terminal"

and verified fixed after adding this patch.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>