fix(ci): fix travis' window build in debian docker by upgrading libseccomp2

seccomp is used by the docker host, and the default version shipped with
Ubuntu 16.04 does not support syscalls used by Qt5's configure inside the
docker container. Upgrading libseccomp2 resolves this issue.

Fix #5874

.travis/build-windows.sh

@@ -108,6 +108,11 @@ ls -lbh "$CACHE_DIR"
 # Purely for debugging
 ls -lbh "$PWD"
 
+sudo apt-get update -qq
+# even though we're building in docker, libseccomp2 is used by docker, and needs to be up to date
+# to support functionality used by Qt's configure
+sudo apt-get install libseccomp2 -y --force-yes
+
 # Build
 sudo docker run --rm \
                 -v "$PWD/workspace":/workspace \