iwd: fix dbus build with pkgconf 1.5.3

iwd: fix dbus 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.

iwd uses the datadir variable from dbus-1.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 usbdropdir to use
explicitly instead of relying on dbus-1.pc.

Fixes:
 - http://autobuild.buildroot.org/results/2b9e28264248f073cc9973bd4c7a89363ec9c0c3

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>