package/squid: use external libltdl

package/squid: use external libltdl

When enabling BR2_PACKAGE_LIBTOOL it depended on the build order if
squid used the external libltdl or built it from its internal copy. That
broke the build in fli4l where we have additional post build checks
analyzing runtime dependencies and a separate step for copying build
artefacts to target packages.

Depend on the external libltdl unconditionally like done with commit
3efc5a250c1c ("package/siproxd: new package") for package siproxd
recently as well. For reproducibility, size, and security.

Link: https://web.nettworks.org/ci/job/fli4l/job/buildroot/job/4.0/job/trunk-sunxi/672/
Suggested-by: Yann E. MORIN <yann.morin.1998@free.fr>
Signed-off-by: Alexander Dahl <post@lespocky.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>