sysutils/grub2-bhyve: Fix build with gcc15

In file included from grub-core/fs/squash4.c:31:
./grub-core/lib/xzembed/xz.h:33:16: error: cannot use keyword 'false' as enumeration constant

33 | typedef enum { false = 0, true = 1 } bool;
   |                ^~~~~

./grub-core/lib/xzembed/xz.h:33:16: note: 'false' is a keyword with '-std=c23' onwards
./grub-core/lib/xzembed/xz.h:33:38: error: expected ';', identifier or '(' before 'bool'

33 | typedef enum { false = 0, true = 1 } bool;
   |                                      ^~~~

./grub-core/lib/xzembed/xz.h:33:38: warning: useless type name in empty declaration

• Add USE_CSTD=gnu11 as a workaround.

PR: 293308
Sponsored by: UNIS Labs
Co-authored-by: Chad Jacob Milios <milios@ccsys.com>