gitlab CI: ensure libtool is installed in build container

Fixes build failure as seen in https://gitlab.freedesktop.org/xorg/driver/xf86-video-nouveau/-/jobs/49933489:

src/Makefile.am:27: error: Libtool library used but 'LIBTOOL' is undefined
src/Makefile.am:27:   The usual way to define 'LIBTOOL' is to add 'LT_INIT'
src/Makefile.am:27:   to 'configure.ac' and run 'aclocal' and 'autoconf' again.
src/Makefile.am:27:   If 'LT_INIT' is in 'configure.ac', make sure
src/Makefile.am:27:   its definition is in aclocal's search path.

(I believe this is showing up now because changes in the gitlab.fd.o infrastructure caused us to regenerate the build containers, and somehow this time libtool wasn't picked up as an automatic dependency of the other packages in the container as it previously had been.)