spandsp: Fix build error with Visual Studio 2019

We don't build spandsp library using MSVC but spandsp plugin is
built by using MSVC.

Part-of: <!692>
16 jobs for master in 68 minutes and 59 seconds (queued for 6 seconds)
Status Name Job ID Coverage
  Build Docker
passed alpine amd64 manifest builder docker #8771405

00:00:23

passed android docker #8771404

00:00:18

passed cerbero fedora amd64 docker #8771387

00:00:22

passed fedora amd64 docker #8771375

00:00:23

passed gst-indent amd64 docker #8771406

00:00:28

passed windows amd64 docker #8771407
windows shell 1809

00:00:09

 
  Preparation
passed manifest #8771408

00:00:11

 
  Build
passed cerbero deps cross-android universal #8771410
gstreamer

00:16:15

canceled cerbero deps cross-ios universal #8771414
gst-ios-14.3

01:08:17

passed cerbero deps cross-windows x86 #8771411
gstreamer

00:13:19

passed cerbero deps cross-windows x86_64 #8771412
gstreamer

00:08:36

passed cerbero deps fedora x86_64 #8771409
gstreamer

00:09:30

passed cerbero deps macos x86_64 #8771413
gst-macos-11.1

01:07:06

passed cerbero deps msvc x86_64 #8771415
docker windows gstreamer-windows 1809

00:51:23

 
  Integrate
passed cerbero cross-android universal examples #8771416

00:07:03

canceled cerbero cross-ios universal examples #8771417
gst-ios-14.3