etc/mtree/BSD.include.dist: Remove libipt-dev

This package only exists on amd64, which means on other platforms we
produce an empty package containing nothing but this directory, and
without a base package (libipt) that package fails to install due to
a missing dependency.

For now, fix this by removing the libipt-dev package tag, which moves
this directory to clibs.

Fixes: 436618a427b4 ("etc/mtree: Add package tags for /usr/include")
Reported by: cperciva
MFC after: 3 seconds