gitlab-ci: add gcc 7.

Change-Id: I3b03c9fcbe67cca5c818f40875dc4828baa05012
Reviewed-on: https://code.wireshark.org/review/23118
Reviewed-by: Alexis La Goutte <alexis.lagoutte@gmail.com>

.gitlab-ci.yml

@@ -65,3 +65,24 @@ gcc-6-nopcap:
     - cd build
     - cmake -GNinja -DENABLE_PCAP=OFF ..
     - ninja
+gcc-7:
+  image: gcc:7
+  script:
+    - mkdir build
+    - cd build
+    - cmake -GNinja ..
+    - ninja
+gcc-7-noextcap:
+  image: gcc:7
+  script:
+    - mkdir build
+    - cd build
+    - cmake -GNinja -DENABLE_EXTCAP=OFF ..
+    - ninja
+gcc-7-nopcap:
+  image: gcc:7
+  script:
+    - mkdir build
+    - cd build
+    - cmake -GNinja -DENABLE_PCAP=OFF ..
+    - ninja