package/vdr-plugin-vnsiserver: 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.

vdr-plugin-vnsiserver uses the locdir variable from vdr.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 LOCDIR to use
explicitly instead of relying on vdr.pc.

Fixes:
 - http://autobuild.buildroot.org/results/9be3719f7b2137a5f039f3c4209c3bc7edeae2b4



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