libaio: allow building on static-only configurations

This commit adds a simple patch to the libaio build system that allows
it to be used on static-only configurations. It is probably easier to
fix this than to propagate the !BR2_STATIC_LIBS dependency.

Fixes:

  http://autobuild.buildroot.net/results/8c86402bd14a0af1b82e6e07c06531a856dd37c6/



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: Romain Naour <romain.naour@gmail.com>
Tested-by: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>