libaio: add host variant

libaio: add host variant

Since the bump of lvm2 to 2.02.180 in commit
8e666bf29e762f3c7681a48d0ee350f8396d798f, lvm2 needs libaio. This was
properly taken into account for the target lvm2 variant, but not the
host lvm2 variant. In order to build the host lvm2, we now need
host-libaio, so this patch adds support for building libaio for the
host.

Part of fixing:

  http://autobuild.buildroot.net/results/f95dd353c17bdfd00fde6762e58aa32e6830b52b/

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>