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>

stubdom/grub/config.h

@@ -2,6 +2,7 @@
 #undef putchar
 #include <ctype.h>
 #include <string.h>
+#include <kernel.h>
 #define debug _debug
 #define grub_halt(a) do_exit()
 #define printf grub_printf

xen/include/xen/libelf.h

@@ -39,11 +39,13 @@ typedef int elf_negerrnoval; /* 0: ok; -EFOO: error */
 #ifdef __XEN__
 #include <public/elfnote.h>
 #include <public/features.h>
+#include <xen/string.h>
 #else
 #include <xen/elfnote.h>
 #include <xen/features.h>
 
 #include <stdarg.h>
+#include <string.h>
 
 struct elf_binary;
 typedef void elf_log_callback(struct elf_binary*, void *caller_data,