snapcast: add libsoxr dependency to snapclient

snapcast: add libsoxr dependency to snapclient

SNAPCAST_SOXR passes -DBUILD_WITH_SOXR, which is a build-wide CMake
option: snapserver and snapclient are built from one source tree and
both link libsoxr. The dependency was declared only on snapserver, so
with SNAPCAST_SOXR enabled (the default) the snapclient package failed
at the packaging step:

  Package snapclient is missing dependencies for the following libraries:
  libsoxr.so.0

Declare +SNAPCAST_SOXR:libsoxr on snapclient too, and relabel the option
"(server + client)" so its build-wide scope is clear.

Fixes: 7673cd9a ("snapcast: make build options explicit and per-package deps precise")
Signed-off-by: Mirko Vogt <redacted>