xenstore: add stddef.h to xenstore_lib.h

xs_perm_to_string takes a size_t which isn't defined by anything
pulled in directly by this header.

Given the other headers xenstore_lib.h pulls in this looks to be an
oversight rather than a deliberate policy.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>

tools/xenstore/include/xenstore_lib.h

@@ -19,6 +19,7 @@
 #ifndef XENSTORE_LIB_H
 #define XENSTORE_LIB_H
 
+#include <stddef.h>
 #include <stdbool.h>
 #include <limits.h>
 #include <errno.h>