gitlab-ci.yml: Install libattr-devel and libcap-devel to test virtio-9p

So far the gitlab-ci was not testing virtio-9p yet, since we did not
install libattr-devel and libcap-devel in any of the pipelines. Do
it now to get some more test coverage.

Message-Id: <20190905111729.1197-1-thuth@redhat.com>
Acked-by: Greg Kurz <groug@kaod.org>
Signed-off-by: Thomas Huth <thuth@redhat.com>

.gitlab-ci.yml

@@ -59,7 +59,7 @@ build-user:
 
 build-clang:
  script:
- - apt-get install -y -qq clang libsdl2-dev
+ - apt-get install -y -qq clang libsdl2-dev libattr1-dev libcap-dev
       xfslibs-dev libiscsi-dev libnfs-dev libseccomp-dev gnutls-dev librbd-dev
  - ./configure --cc=clang --cxx=clang++ --enable-werror
       --target-list="alpha-softmmu arm-softmmu m68k-softmmu mips64-softmmu