stubdom: Fix stubdom undeclared function build warnings

stubdom: Fix stubdom undeclared function build warnings

This includes a few headers to fix some missing function declarations.

../grub-upstream/stage2/builtins.c:1728:3: warning: implicit declaration of function ‘do_exit’ [-Wimplicit-function-declaration]
stubdom/include/xen/libelf/libelf.h:453:5: warning: implicit declaration of function ‘memcpy’ [-Wimplicit-function-declaration]

Reported-by: IAN DELANEY <della5@iinet.com.au>
Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
Acked-by: Ian Campbell <ian.campbell@citrix.com>