build: Unconditionally dist tapset files

Apparently whoever made the last tarball didn't have `--enable-systemtap`.
Disted files shouldn't depend on build flags.

Also, it is cathartic for me to say tarballs are dumb and we should
just be pulling from git which is what's canonically tracking what's
source code and what's not and we don't need to repeat ourself in
`Makefile.am`.

https://bugzilla.gnome.org/show_bug.cgi?id=762637