grub: Fix build with gcc-6
Backport patch which silences following
'../../grub-2.00/grub-core/'`gfxmenu/model.c
../../grub-2.00/grub-core/gettext/gettext.c:37:36: error: storage size of 'main_context' isn't known
static struct grub_gettext_context main_context, secondary_context;
^~~~~~~~~~~~
make[3]: *** [gettext/gettext_module-gettext.o] Error 1
(From OE-Core rev:
4efac9861ab59d696bdc81ea59497febfa2d0dc8)
Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
