package/xen: Include linux/limits.h when building QEMU inside of Xen

If building Xen and not using glibc (using musl for example) the QEMU
project inside of Xen fails to build as it relys on the XATTR_SIZE_MAX
macro which is defined in <linux/limits.h>.

Glibc includes this header indirectly but as that is not part of the
POSIX standard other libraries don't neccessearily have this. To fix the
issue we explicitly include <linux/limits.h> to ensure the build works
for all libc libraries.

Fixes:
  http://autobuild.buildroot.net/results/bb1/bb1310eb1464d880a357ae5e491fd168361d25a0



Signed-off-by: Alistair Francis <alistair.francis@xilinx.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>