gitlab-ci: Add pyelftools when needed

In order to mirror current Travis CI support we need to install this
package via pip.
Signed-off-by: NTom Rini <trini@konsulko.com>

.gitlab-ci.yml

@@ -22,6 +22,7 @@ stages:
     - . /tmp/venv/bin/activate
     - pip install pytest==2.8.7
     - pip install python-subunit
+    - pip install pyelftools
     - grub-mkimage -o ~/grub_x86.efi -O i386-efi normal  echo lsefimmap lsefi lsefisystab efinet tftp minicmd
     - grub-mkimage -o ~/grub_x64.efi -O x86_64-efi normal  echo lsefimmap lsefi lsefisystab efinet tftp minicmd
     - mkdir ~/grub2-arm