package/pulseaudio: add optional support for libsoxr

When libsoxr was compiled before, pulseaudio will use it as optional
dependency:

$ output/host/usr/bin/x86_64-linux-readelf -a output/target/usr/bin/pulseaudio | grep NEEDED
 0x0000000000000001 (NEEDED)             Shared library: [libpulsecore-8.0.so]
 0x0000000000000001 (NEEDED)             Shared library: [libpulse.so.0]
 0x0000000000000001 (NEEDED)             Shared library: [libspeexdsp.so.1]
 0x0000000000000001 (NEEDED)             Shared library: [libsoxr.so.0]
[...]

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>