travis: added libzstd-dev (completes https://trac.osgeo.org/grass/changeset/74391)

git-svn-id: https://svn.osgeo.org/grass/grass/trunk@74392 15284696-431f-4ddb-bdfa-cd5b030d7da7

.travis/linux.install.sh

@@ -14,6 +14,7 @@ sudo apt-get install --no-install-recommends \
     libfftw3-dev \
     libfreetype6-dev \
     libgdal-dev \
+    libzstd-dev \
     libgeos-dev \
     libglu1-mesa-dev \
     libjpeg-dev \