package/libsoxr: add avutil to soxr.pc

Add ${AVUTIL_LIBRARIES} to soxr.pc.in so applications such as
shairport-sync will know that they must link with -lavutil when
building statically

Fixes:
 - http://autobuild.buildroot.org/results/839c0ce6475accc1de7e8a180d4358edb6750c64



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
[Thomas: change patch to use Libs.private instead.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>