build: fix distcheck when building settings specification XML

build: fix distcheck when building settings specification XML

The generated settings-spec.xml should not be DIST-ed because it's
regenerated every time libnm-util changes.  That meants it gets
rebuilt during 'make' even from a dist tarball.  Thus, it shouldn't
be part of the dist tarball itself.

In addition, settings-spec.html shouldn't be included in
$(content_files) becuase this is added to EXTRA_DIST by gtk-doc.make.

Since generate-settings-spec.c the move from docs/api/ to tools/,
this started breaking distcheck even though the original Makefile
rules were wrong too.