libv4l: fix udev build with pkgconf 1.5.3

libv4l: fix udev 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.

libv4l uses the udevdir variable from libudev.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 udevdir to use
explicitly instead of relying on libudev.pc.

Fixes:
 - http://autobuild.buildroot.org/results/597cd814225c2af9b896a1f25f98cdff33fd810a

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