gitlab-ci: upgrade before anything else in fedora28.

Fix a bug in linking:
/usr/lib/gcc/x86_64-redhat-linux/8/../../../../lib64/libgnutls.so: undefined reference to `nettle_rsa_sec_decrypt@HOGWEED_4'

Change-Id: Icc85813bd72274e448667bab4a80037ec5bdd48d
Reviewed-on: https://code.wireshark.org/review/31519
Reviewed-by: Dario Lombardo <lomato@gmail.com>

.gitlab-ci.yml

@@ -96,7 +96,8 @@ rpm-opensuse-42.3:
 rpm-fedora-28:
   image: fedora:28
   script:
-    - yum install -y which
+    - dnf upgrade -y
+    - dnf install -y which
     - tools/rpm-setup.sh --install-optional -y
     - mkdir build
     - cd build