Move webvtt-config.h to the top-level build directory.

include/webvtt/ doesn't exist in out-of-tree builds, so
this was breaking 'make distcheck'. Placing it beside
the also-generated config.h is a work-around.

Another approach would be to copy all the installed headers
into to the build tree, but that seems more confusing.
Clean up a spurious tab.
Add missing files to the distribution list.

This gets autotools 'make distcheck' as far as the disabled
tests failing. Everything that's needed by the build needs
to be listed in the Makefile.am files, either as _SOURCES,
noinst_HEADERS, or as EXTRA_DIST.

There may be other missing files; I didn't do a complete check.