Fix install of generated Xcursor.h when builddir != srcdir

Signed-off-by: Jon TURNEY <jon.turney@dronecode.org.uk>
Reviewed-by: Gaetan Nadon <memsize@videotron.ca>
Tested-by: Gaetan Nadon <memsize@videotron.ca>
Reviewed-by: Alan Coopersmith <alan.coopersmith@oracle.com>