Makefile: add missing DESTDIR variable use
    
    Patch borrowed from OpenEmbedded, available at
    recipes/libaio/libaio-0.3.106/destdir.patch in their source tree.
    
    It just adds support for the traditional DESTDIR variable to install
    the library in a different sysroot than the normal /.
    
    Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
    [JEM: I couldn't verify the initial author of this change, but it's
     trivial.  For information on the DESTDIR convention, see:
     https://www.gnu.org/prep/standards/html_node/DESTDIR.html]
    Signed-off-by: Jeff Moyer <jmoyer@redhat.com>