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: Tom Rini <trini@konsulko.com>
diff --git a/.gitlab-ci.yml b/.gitlab-ci.yml
index cb636b2..da27cae 100644
--- a/.gitlab-ci.yml
+++ b/.gitlab-ci.yml
@@ -22,6 +22,7 @@
     - . /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