Many capitalization can be found for this library (spandsp, Spandsp, SpanDSP),
let's use the one found in the library README and in its spec file.
Change-Id: Ia66b723e5d582a6218da1b6366b7d4859272f80c
Reviewed-on: https://code.wireshark.org/review/19122
Reviewed-by: Peter Wu <peter@lekensteyn.nl>