mkimage: fix build regression in grub_mkimage_load_image

mkimage: fix build regression in grub_mkimage_load_image

The grub_mkimage_load_image function (commit 7542af6, mkimage: refactor a bunch
of section data into a struct.) introduces a build regression on SPARC:

  cc1: warnings being treated as errors
  In file included from util/grub-mkimage32.c:23:
  util/grub-mkimagexx.c: In function 'grub_mkimage_load_image32':
  util/grub-mkimagexx.c:1968: error: missing initializer
  util/grub-mkimagexx.c:1968: error: (near initialization for 'smd.sections')
  make[2]: *** [util/grub_mkimage-grub-mkimage32.o] Error 1

Initialize the entire section_metadata structure.

Signed-off-by: Eric Snowberg <eric.snowberg@oracle.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>