kmod: fix build with pkgconf 1.5.3

kmod: 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.

kmod uses the completionsdir variable from bash-completions.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 bash-completions.pc.

Fixes:
 - http://autobuild.buildroot.org/results/f8a1f956333062027294e766ff0ddab5c35d5887

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