grub2: fix compile error with glibc 2.19.1

BUG=chromium:400426
TEST=grub compiles with glibc 2.19.1

Change-Id: Icef6829461ab037f356b66720f788617ac165da1
Reviewed-on: https://chromium-review.googlesource.com/211001
Reviewed-by: Mike Frysinger <vapier@chromium.org>
Commit-Queue: Yunlian Jiang <yunlian@chromium.org>
Tested-by: Yunlian Jiang <yunlian@chromium.org>
1 file changed