xapp_xdm: fix build with pkgconf 1.5.3

xapp_xdm: fix build with pkgconf 1.5.3

Since the bump to 1.5.3, pkgconf prepends the sysroot to all absolute
paths found in the .pc file. This is correct when the paths refer to
something in STAGING_DIR (e.g. libdir, includedir), but not when it
refers to something used for the target.

xapp_xdm uses the appdefaultdir variable from xt.pc to decide where to
install things. Since DESTDIR is prepended to the install destination,
this will end up in the wrong location.

Until a better solution is found in pkgconf, pass the appdefaultdir to
use explicitly instead of relying on xt.pc.

Fixes:
 - http://autobuild.buildroot.org/results/08bcba7d7340f34dc66b5b2ab8fbcfbaee309e37

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>