libmatroska: add host-pkgconf dependency

libmatroska configure.ac uses PKG_CHECK_MODULES(), so this package
needs to depend on host-pkgconf.

Fixes:

  http://autobuild.buildroot.org/results/80a/80a9f0f5594ab833600ea8514dc184e50d2bf08c/



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>