grub2: set -std=gnu17

grub2: set -std=gnu17

Arch Linux updated to GCC15 which now defaults to GNU23 as the default
instead of GNU17[1], and this breaks grub compilation.

This is fixed in more recent grub2 versions.

[1] https://gcc.gnu.org/gcc-15/porting_to.html#c23

Link: https://github.com/openwrt/openwrt/pull/19065
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>