xen: tidy up whitespace in drivers/xen/Makefile

Various merges over time have led to rather a mish-mash of indentation.
Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Cc: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Cc: xen-devel@lists.xensource.com
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>