doc: fix distclean

doc: fix distclean

Apparently Automake has some undocumented logic somewhere that makes it
so any Makefile generated from an Automake Makefile.am is removed from
its secret list of things to delete that it deletes when performing a
recursive distclean before actually performing the recursive distclean
and since the secret list is automatically generated from the list of
things that Autoconf should generate in configure.ac we can't remove the
Makefile from that list or it will break Automake's list of things to
automatically generate that it generates from Autoconf's list of things
to automatically generate.

Thus, to prevent Automake from deleting Makefiles and then immediately
trying to use the Makefiles it just deleted to delete said Makefiles, we
must remove ourselves from the secret list, which is accomplished by
changing the file extension to '.am' instead of '.in'.

Signed-off-by: Quentin Young <qlyoung@cumulusnetworks.com>