libaio: make available only on supported architectures

libaio has some architecture-specific code, and isn't available for
all architectures that Buildroot support.

Fixes
http://autobuild.buildroot.org/results/b171a6d6243ea021dc52830bc4b9d36fc083a60d/build-end.log

.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>