package/shairport-sync: add upstream patch to fix build issue with soxr

package/shairport-sync: add upstream patch to fix build issue with soxr

Add upstream patch to fix build issue detected by the Buildroot autobuilder:

```
arm-buildroot-linux-uclibcgnueabi/sysroot/usr//lib/libsoxr.a(soxr.c.o): In function `soxr_create':
soxr.c:(.text+0xd4c): undefined reference to `av_get_cpu_flags'
collect2: error: ld returned 1 exit status
```

A quick look at the soxr source code shows that `soxr_create()` might use
`av_get_cpu_flags()` depending on the architecture.

For the sake of simplicity link with `-lavutil` if it is found when using soxr.

Even better, as soxr provides a pkg-config file, this should be prefered.

Upstream issue: https://github.com/mikebrady/shairport-sync/issues/733
Upstream status: 5101ab2d13e2b89ea3c1276df5fb7413634eeccd

Fixes:
http://autobuild.buildroot.net/results/53d/53d21686780aa2485745b59e812b6280dd39f1c5
http://autobuild.buildroot.net/results/605/60576363adfca404c3a7883d5d46e8a4a9ee8171
http://autobuild.buildroot.net/results/806/806867ab1c6f42ad1b34d44844efc57272d48235
http://autobuild.buildroot.net/results/840/840810601fbb8a7957ea9dae175b959a7f9b7491
.. and more.

Signed-off-by: Jörg Krause <joerg.krause@embedded.rocks>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>