travis: add s390x builds.

Change-Id: Ib5460beffd185c5d5126302001701125955b973e
Reviewed-on: https://code.wireshark.org/review/35079
Reviewed-by: Anders Broman <a.broman58@gmail.com>

.travis.yml

@@ -45,6 +45,11 @@ matrix:
         - CXX=g++-8
         - CC=gcc-8
         - PCAP=ON
+    - name: Linux s390x (gcc-7) with PCAP
+      os: linux
+      arch: s390x
+      env:
+        - PCAP=ON
     - name: Linux (gcc-8) without PCAP
       os: linux
       addons:
@@ -70,6 +75,13 @@ matrix:
         - CXX=clang++
         - CC=clang
         - PCAP=ON
+    - name: Linux s390x (clang) with PCAP
+      os: linux
+      arch: s390x
+      env:
+        - CXX=clang++
+        - CC=clang
+        - PCAP=ON
     - name: Linux (clang) without PCAP
       os: linux
       env: