package/xen: Use POSIX complaint header includes

package/xen: Use POSIX complaint header includes

To fix build issues when using the musl library use POSIX compatible
library inclues.

This fixes this autobuilder issue:
http://autobuild.buildroot.net/results/1aa/1aa1303f60372f51aa5a7eb18caac4a5b5c1d9d4/build-end.log

These two patches have been by accepted upstream Xen and will be in 4.9:
http://xenbits.xen.org/gitweb/?p=xen.git;a=commit;h=b4cd5173183fbc118e2dc2a0d2e0d5038daf4fb5
http://xenbits.xen.org/gitweb/?p=xen.git;a=commit;h=4703a9ba9bb0c9c2804813ffe0943177d5f96039

Signed-off-by: Alistair Francis <alistair.francis@xilinx.com>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>